Prejudice is one of the world's greatest labor-saving devices; it enables you to form an opinion without having to dig up the facts. -Laurence Peter

Myth: Vegans don't get enough protein (aka- "The Protein Myth") (link)
Truth: Vegans easily get enough plant-based protein in their diets
America has an obsession with protein, and the first question usually a vegan is usually asked by an uninformed omnivore is where their protein comes from. In the well-intentioned hope of becoming "healthy" most people forget that anything other than traditionally proteinaceous foods such as meat contain protein. On the contrary, almost all plant-based foods contain protein, and as you can see in dietary comparisons such as the one at SoyStache, a vegan diet offers TONS of easily obtained protein sources. It's nearly impossible to become protein deficient if you're eating a varied diet with sufficient caloric intake.
Also, you don't risk adding dangerous levels of cholesterol into your body, since a plant-based diet is 100% cholesterol-free, and unlike most omnivorous diets you'll be getting a more realistic protein intake. Too much protein in your diet causes calcium to be leeched from your bones (which is interesting in relation to "The Calcium Myth" seen just below) and puts you at risk of kidney disease, kidney stones, colon cancer, osteoporosis, and other ailments.
Additional Resources: VRG, PCRM, EVU, VFL. For a metaphorical look at "The Protein Myth" check out this article. To learn about the big fat lies behind the Atkin's diet (and other dangerous high-protein fad diets) look at this PDF article (It's worth the download).

Myth: Vegans don't get enough calcium (aka- "The Calcium Myth") (link)
Truth: Vegans easily get enough plant-based calcium in their diets
This myth you can pretty much thank the dairy industry for. Thanks to years of dairy-sponsored "health information", and the blatant lies behind the "Got Milk" ad campaign, people seem to be more convinced than ever that sufficient calcium only comes from cow's milk. Like protein, it's almost impossible to become calcium deficient if you're eating a varied diet with sufficient caloric intake. Take a look at the dietary comparison of calcium in foods at the SoyStache site and you'll see how easy it is to get sufficient calcium in your diet almost without trying! Like animal-based foods rich in protein, dairy-based foods are loaded with cholesterol and are known to cause anemia, food allergies, cancer, heart disease, diabetes, and ironically enough, lactose intolerance and osteoporosis. Interestingly, studies have shown that due to high protein content, dairy consumption actually helps cause calcium loss!
For healthy bones and a healthy calcium intake you can switch to a plant-based diet, quit smoking, make sure to exercise, avoid excess salt, and get your vitamin D from the sun. (If you're feeling extra motivated, learn about phosphorus ratios in food and how they effect your blood's pH balance and calcium usage.)
Additional Resources: VRG, PCRM, NotMilk, PETA.

Myth: A vegan diet is a health compromise (link)
Truth: A vegan diet is the healthiest choice possible for almost anyone
For many this is a deeply rooted prejudice, even among the vegetarian and vegan community. In the US we are constantly inundated by misleading claims made by the meat and dairy industries, and after a lifetime of such indoctrination even the most well-intentioned veggies can find themselves worried that their diet may negatively affect their health. Like all myths, just a little research proves this to be false.
While there are always exceptions to the rule, human bodies flourish when eating a well planned plant-based diet. (A poorly planned vegan diet is usually the scapegoat for people who try vegetarianism/veganism but "don't feel healthy". It should go without saying, but any diet should be thoroughly researched before undertaking.) The goal of eating is to obtain certain vitamins, minerals, proteins, carbohydrates, etc. and to get sufficient calories to sustain our activities. When you look at eating in such an abstract way, and give up any preconceived notions about which specific foods seem to be the only way to obtain those certain essential nutrients, you're ready to really understand nutrition. The 4 food groups were created by the USDA so people didn't have to worry about the specifics of their nutrients. Instead of looking at a huge table of what foods contain what necessary nutrients, people were able to simplify things by looking at a table with just 4 sections, each with a recommended daily dose of foods. While this may simplify our lives, it's also trained us to think of nutrition as FOODS instead of NUTRIENTS.
When you look at what nutrients are contained in plant-based food compared to what our bodies need, it's obvious that a vegan diet is plentiful in good stuff (vitamins, minerals, fiber, etc.) and low on the junk (saturated fats, cholesterol, hormones/antibiotics, etc.). Animal-based sources of the good stuff (protein, calcium, etc.) are also loaded with the junk, a problem vegans don't have to worry about. (The only deficiency vegans are at risk for is the vitamin B12. Although, when you discover B12 is a bacteria and animal-based foods contain B12 only due to bacterial contamination, it makes taking a supplement sound like a smart idea all around)
One of the main precepts of a vegan lifestyle is to do the least amount of harm, and that most definitely includes oneself! A varied vegan diet is not just good for animals and the earth, but is the absolute healthiest possible diet for humans.
Additional Resources: Vegan Society, PCRM, American Dietetic Association, VeganHealth.com

Myth: Vegans eat weird/unappetizing/hard to find and prepare foods (link)
Truth: Vegans have a wide variety of "normal", and delicious food choices
This idea comes from the "different means wrong" school of thought and can prevent everyone from discovering new foods that they might find very tasty. In any typical American dining setting there is usually a well known selection of foods that are vegan safe, but since a plant-based diet eliminates many foods, vegans often become more open minded to new food options. (A good example is nutritional yeast, which is not commonly found in an omnivorous diet. It has a cheesy, nutty taste and can be used for anything from a thickener, a condiment, or an excellent "cheese" sauce. I much prefer nutritional yeast "mac & cheese" to traditional mac & cheese, but I might have missed out if I was afraid of trying "weird" foods.)
Many ethnic restaurants such as Thai, Chinese, Vietnamese, Indian, and Ethiopian (to name a few) have TONS of vegan options and will widely broaden the range of food you enjoy. At fast food places vegans can order veggie subs, bean or veggie burritos or wraps, french fries, baked potatoes, fresh fruit, steamed vegetables, pasta with marinara sauce, fruit smoothies, veggie burgers, fruit turnovers, and salads with Italian or vinaigrette.
At home (or at a veg friendly establishment) a vegan's choices are unlimited. Like those with omnivorous diets, not all vegans eat the same way. Some do eat a lot of non-traditional foods like tofu, other soy products, but many vegans eat a lot of traditional, "normal" foods. It's common to take an almost-vegan dish, such as cheese lasagna or bakery items, and replace the offending ingredients with plant-based ones instead. Surprisingly these newly renovated favorites can end up tasting very similar to their meat/dairy/egg containing counterparts. There are even vegan chefs, upscale vegan feasts, and vegan fast food restaurants that help prove that plant-based cooking can be every bit as yummy, creative, and easily obtainable as any traditional foods. Vegans can also easily do their shopping in any chain grocery store, and whole/health food stores provide lots of additional choices.
Additional Resources: VRG Guide to Fast Food, Fast Food Nation, Buy vegan treats!

Myth: Vegans (and/or Animal Rights activists) are violent terrorists (link)
Truth: Vegans are humans and come in all degrees of non-violent and violent tendencies
Vegan is not synonymous with violent. We're not all ALF members. Since veganism fosters a respect for all life (not just animals), it is absurd to come to the conclusion that violence is the only method a vegan would use to affect change. This is also a hypocritical stereotype, in that vegans choose to actively lessen the violence and murder in our world, while omnivores continue to support it.
That being said, one method of attempting to bring about change, for vegan and non-vegan causes alike, is called direct action. Rather than symbolic actions (like protesting) or indirect actions (like letter writing), direct action dives headlong into the problem. In the animal rights community an example of direct action could be breaking into an animal testing lab to free animals and maybe even cause some financial hardship by destroying equipment or property. I am not condoning or condemning this form of activism, simply clarifying. This sort of direct action is used not just by "butcher bombing" vegans (I've actually heard this), but by defenders of civil, environmental, political, gender, and other rights. Being a vegan does not mean a person is involved with animal rights, and definitely does not mean they participate in violent acts.

Myth: Vegans are all ____ (insert slur here) (link)
Truth: Vegans come from all different walks of life, and despite their unifying vegan beliefs, we all live varied lives.
Lately I've been seeing more of this stereotyping, but it seems to be directly in proportion to the increased exposure of veganism to the omnivorous world. Rather than learning what veganism encompasses, some people feel it should be written off entirely and summarized with one-liners. Commonly overheard slurs include "all vegans..."; eat tofu, are hippies, deify animals, don't change anything, are non-violent, are weak and/or malnourished and/or very thin, are tree-huggers, know less about nutrition than omnivores, eat only a few foods... and the list goes on. It's true that, by definition, vegans (not to be confused with total-vegetarians or vegetarians) have made their lifestyle choice because of moral reasons. We all share the common thread of wanting to do "the least amount of harm and the most amount of good". Beyond that vegans are as different as any other group of individuals! We come in different body sizes, political affiliations, occupational backgrounds, and religious denominations. We eat widely varied diets, have different levels of environmental participation, different outlooks and goals in life, any aspire to be many things. Lumping all of the millions of vegans together is as intelligent as racism, sexism, or ageism. (But then again so is that lifestyle some people defend!)

Myth: Vegans think animals are the same or better than humans (link)
Truth: Vegans recognize that animals are sentient beings and afford them the respect that deserves
In the same way that parents do not have a favorite child, vegans do not all prefer animals to all humans. People are quite able to focus their concern not just on their own species (a novel concept!), but to branch out in other directions. Additionally, each person has their own set of innate characteristic which leads them in those other directions. An omnivore (or vegan) might have lost a loved one to cancer, and so their desire to help others might naturally lead them to get involved with a cancer research organization. Some vegans, after learning that there are virtually no Federal laws protecting "food animals", may be compelled to help farm animals. It is a matter of a) what we as individuals feel moved by, and b) what we are exposed to. Each person must decide for themselves what their "callings" are, and as long as they do not harm themselves or others, these callings should not be trivialized. But like omnivores, vegans have a myriad of interests. Just because a person cares for the welfare of animals does not mean that person isn't active in causes of a political, environmental, spiritual, or humanitarian nature.
And let's be serious here. Animal rights activists aren't asking for perks like animal day care, or subsidized housing. They're simply trying to get animals the respect that any feeling creature deserves- not to be brutally abused, slaughtered, or exploited for any reason.

Myth: Eating vegan is expensive (link)
Truth: Vegetables are cheaper than meat, period. It's processed, pre-prepared food (of any content) that you have to watch out for.
Admittedly, not the worst myth I've ever heard, but a myth nonetheless. Two major points on this. First, the true cost.
Everything has a cost, whether it's monetary, emotional, or physical. Pretty much everyone can agree that money is not the most important thing in life, and yet we let monetary cost outweigh our concern for other expenditures.We buy conventionally grown produce because it's monetarily cheaper, even though we pay the price with barren, insecticide contaminated land. We buy shoes at Payless because we save a few dollars, even though sweatshop workers pay with their entire lives on the other side of the globe.
Did the Grinch just effing steal Christmas?
There's some foods that are sometimes more expensive than their naughty counterparts (organic produce vs. conventional product or vegan cheese vs. dairy cheese, for example), but it's just not that hard to make some priorities higher on the list than money. But for those of us who like to be thrifty or are low on money in the first place, there's point number 1.5.
Vegetables and grains are cheaper than meat, period. Plants grow from the earth. When they're ripe, they're ready to go. When meat is produced you start with plants grown from the earth, and when they're ripe and you pick them. Theeeen you feed them into an animal and fatten them up (insert wicked laughter here), and then you kill it and it's ready to go. You can stop paying the middle man anytime you're ready, and just stick with the food that comes directly from the earth. You could even start buying those exotic, expensive looking fruits you see in the specialty produce section instead of blowing several bucks a pound for meat. Picking all the right, yummy, inexpensive vegan foods is an art, but one you can easily master with a little time and courage to try new things.
Point two is looking at your lifestyle to see if you're demanding a high level of food maintenance. Do you need your food to come in a box that just needs water added? If it's not ready in 10 minutes or less from your microwave, is your dinner just too much work? Sorry, but you must get over it! No matter what diet you follow, pre-prepared food is expensive (not to mention unhealthy). Tofurkys and TV dinners cost more than veggies and grains. If you're not married to the idea of eating vegan versions of all the old garbage you used to eat then you will reduce your food budget. Fake hot dogs, vegan marshmallows, and soy deli slices are great treats, but not for every day. You can be thrifty and vegan, for sure.
Additional Resources: Cheap Vegan Zine, Cheap Vegan LJ community