I can't knit. I desperately wish I had the time/patience/I swear to god natural born talent some of these knitters have, but for right now, I don't. If I could knit, this might just possibly be the only form of public art I would ever be interested in helping along.
Granny-style graffiti? Sign me up. (Okay, so I'm fairly certain this bus was commissioned but there are some definite graffiti examples in their gallery.) More here.