-People who don't use their turn signals. If you're going to make a lane change or a turn, I'm sure it wouldn't be too difficult to flick a little stick. Who knows? It might avoid accidents!
-Cellphones! I can't stand it when a driver keeps swerving because they're talking on their cellphone. At least a headset would be nice, if you really must talk on the phone.
-Tailgaters. It's not safe for them, and it's not safe for the other cars around them.
-People who have dinner behind the wheel! I don't have anything against eating behind the wheel, but if it's going to really distract you, stop! I had the misfortune of riding in a car whose driver was munching on food and taking his eyes off the road frequently. The worst part was that the road was twisting and turning at the edge of cliffs and whatnot. I believe it was California Route 1, but I'm not too sure. Anyways, I was ready to jump out into the ocean.

That was quite some time ago though.
Jam it back in, in the dark.