I dunno if this really counts as a hobby, but I spend a lot of time on things like Wikipedia reading about things that catch my interest- It kills some time at least, but careful with it, sometimes that lost time sneaks up on you later haha.

Other than that I like to sketch stuff, play guitar, read (Though I have to be in the right frame of mind and have a large block of time), cycling and mountain biking, skateboarding (not so much anymore), tweaking electronics, and worrying about stupid stuff; (Not the best hobby though, so take heed.).