Just a few days ago finished TES3: Morrowind. Even though I mostly just went straight through the story with a few sidequests along the way, it took me a good few weeks of work to finish. When it comes up on the Steam sales I'm going to get the GOTY edition and recreate my XBox character with console cheats.
I've also been trying out an old gem, X-Com: UFO Defense. It's been a blast so far, there's lots of tactical depth that's been lost on modern strategy games IMO. I can only hope the new game measures up in comparison!