One word: Morrowind
Three words: sucked my life
Two years ago, Morrowind came out for the Xbox. That was the impetus for my purchase of the Xbox. I went out and bought an Xbox + Morrowind. And then I proceeded to dump hour upon hour upon day... for many weeks. I really don't know how many hours that I put into the game. Hundreds. If I counted, then I couldn't dodge the question when somebody asked me just how insane I was. "A lot" is way better than "oh, about 300 hours"
In a nutshell, Morrowind defines freedom. There is no other game on the planet that has its open-ended play. GTA? Not even close. I spent a ton of time just doing side-quests and character development, rather than following the main storyline. Eventually, I returned to the story, but only after having a seriously kickass time.
Fable was also slated to have similar open-ended play, although I recently read that it may have been narrowed into a little more linear scope. We'll see when it ships. I'm really waiting for that one. For the intervening months, I think that I'll occupy myself with Sudeki which comes out in July.
If you haven't played Morrowind yet, then I'd highly recommend the Game of the Year edition. It's got all the extra goodies. And for thirty bucks? Hell yah.