I'd like a paint-eating liquid that could also be carried in a water bottle. Squirt car, driver doesn't get too mad as it's only water...until minutes later when you're gone and so is their paint job.
Ok, so I'm dreaming and really, the effort it would take to prepare and use such a weapon would out weigh just ignoring dumbass drivers.
I've heard from motorcycle-riding friends that a well placed foot will easily take off a side mirror. Once again, this is probably more fun to think about than to actually do.