I have car magnets that I've used in the past. I know others on here have used them as well with good results. Someone from our Small Business Development Center told me that this is one of the most effective forms of advertising. She also mentioned though that it's best to put them on the back of your car rather than the sides to be the most effective. My car only had room on the sides, but I still got some inquiries from them.
I got mine from Vista Print last summer. I think I only paid about $20 for two of them (the smaller sized ones.) I don't usually get much from Vista Print, but these were a pretty good deal and they've stood up pretty well. One of them even survived a drive thru car wash.
You can find another discussion on car magnets
HERE.