I shot a man in Reno just to watch him die...
Other than that I've slapped a couple cars on the trunk and once lobbed a nice loogie into the bed of a pickup that almost ran me off the road. The pickup bed was full of tools and the guy saw what I did (I did it when I caught up to him at a stoplight). By that point there wasn't much he could do since he was stuck in traffic and I was merrily riding on my way...