Morrowind. It's a HUGE rpg (lots and lots bigger than, for instance, fallout 2 or baldur's gate) which gives you almost complete freedom to do whatever the fuck you want. You can become a mighty warrior or a sweet-talking assassin, or even a wanted outlaw or a vampire, feeding on the blood of innocent maidens
The game is entirely non-lineair, so you can choose to dabble in being a hero and saving the world, or just build your own personal fortune and become the leader of some of the many factions in the game... or just alternate between the two.
Best game ever, if you like RPGs
Edit: If you choose to do everything in the game, I believe it takes about 400 hours of playing, and there are two expansion packs which add another 100 or so hours. Besides that, it's also highly addictive, so if you value your life and don't want to waste a few months playing a game, you might want to avoid it like the plague
