I used to waste a lot of time on Tumblr but I quit it.
I pick at hangnails and scabs and stuff a lot.
I spit almost constantly while outside (used to play baseball and I guess that rubbed off).
I drink 2-3 bottles of soda a day on average.
I used to watch too much porn, but I've worked on quitting that too.
That's it as far as bad habits I think.
Cracking your knuckles literally does nothing. I do it multiple times a day all day long and I'm a guitarist and I've never experienced any pain related to cracking my knuckles while playing or practicing.
Oh yeah, I bounce my legs a lot while sitting. That's not really a bad thing though, it just annoys some people. And I talk to myself a lot but that's not really a bad thing either since I'm just vocalizing my own thoughts to myself.