Listening to music. I listen to it practically all day long, either in the background to relax, or right up front and loud to enjoy it if nothing else is going on. I even have it playing while I'm doing homework, that way I can't hear the TV and get distracted, and I can actually concentrate better.
This is just a recent development, though, which is weird. I used to rarely listen to music, except for playing classical music on the piano and sometimes every so often listening to it.
ITunes reports 5:15:50 worth of music (48 songs). Should be improving that soon.