Hummers and other unnecessarily gargantuan SUV's. They are incredibly impractical, douchy, and tacky as fuck. If you have that kind of money to spend on a car at least buy something with some class, not an overpriced chevy tahoe that looks like it was designed using legos.
Hybrids (Priuses especially) because they are driven by pretentious fags who want to look eco-conscious, but didn't actually do their research. Clean diesels are more fuel efficient and overall less harmful to the environment.
Ricer morons who put giant spoilers on their FWD cars. Also generally tasteless modding of cars really annoys me. Especially when people ruin German cars with ricer crap.
Smart cars, the Nissan Cube and such don't really bother me. I agree they are pretty gay looking but I can respect that somebody bought it in the name of practicality and frugality, even knowing how much of a homo they are going to look like.