I always get a laugh out of vanity license plates, although sometimes I'm actually laughing AT them and their owners. Here are a couple of good ones:
1. My sister was telling me about her co-worker who loves the Dave Matthews Band. Additionally, the co-worker has recently become a mom, so she decided to get a vanity plate that combines the two: DMB MOM
Unfortunately, many people misinterpret it as "Dumb Mom." D'oh!
(Amy, feel free to correct any mistaken details on that one.)
2. The best one I've seen recently was a litte red sportscar with the license plate PRIME 8. That alone might be amusing, but it was greatly accentuated by the fact that the driver had a real live monkey (a rhesus monkey or something similar) sitting on his lap! I did a double take. I thought it was pretty awesome -- until I realized that the guy and his monkey were taking advantage of the HOV (i.e., carpool) lane! That just doesn't seem right.