As much as I hate to admit it... World of Warcraft. I'd been playing since the final stress test and it then proceeded to feed off my soul day and night for the next four years. After many unsuccessful attempts to quit, I came to my senses and erased every semblance of that game from my computer, deleted all WoW related bookmarks, etc. I've been sober ever since. It's difficult to stay completely away though since my brother got a job working for Blizzard last year.