10. "No real programmer would ever use PHP." - This won't work for every geek, of course, but for those it works on, it should work really well.
9. "Comic books are just for kids!" - I'm sure you've heard this one before—I know I certainly heard it often enough in high school, and even though it's even less true now than it was then, I'm sure comic book afficionados still hear it today.
8. "Role-playing games are just for people who can't deal with real life." - There are, sadly, still a lot of people who think anyone who plays D&D must live in his parents' basement and bathe once a month. Such people must be put straight, and immediately!
7. "The Pirates of the Caribbean movies are so realistic!" - I doubt many people actually believe this, but there are an awful lot of misconceptions about pirates out there, so you never know.
6. "Yeah, I got an Xbox 360 so my daughter could play Hello Kitty games. Is it really good for anything else?" - We've hit the ones that are hard for me even to type, now…
5. "Mac, Windows, or Linux? Does it really make a difference?" - An argument so old its original form was probably first written down in hieroglyphs, I know, but I don't know a single geek it wouldn't work on—myself included.
4. "The Ewoks were the best thing about the original Star Wars trilogy." / "Greedo shot first!" - I couldn't decide between the two. If one doesn't work, I'll bet the other one would.
3. "Tolkien? Ehhh, I prefer Terry Brooks!" - I almost feel like I should argue with myself just for writing that. I'm going to let the top two stand for themselves.
2. "Joss Whedon is a hack!"
1. "I don't see what's so bad about DRM!"
Any other good ones come to mind for you? Please share it in a comment!