RoryB briefly mentions his annoyance with people who drive slow in the left lane, which is the passing lane. I totally agree with him, but I have another beef with people: not signalling. Drive like an ass, if you want, but at least signal to me that you are going to cut me off. I'll be the first to admit that I drive fast, but I always signal when I'm switching lanes; it is just smart driving, letting everyone else know that your intentions.
According to a USA Today snapshot poll, my peeve is more common than Rory's, although not by much:
[source: USA Today Snapshot]