Thanks to nexusmods.com I've been replaying the Bethesda open world games (Morrowind, Oblivion, Skyrim, Fallout 3, Fallout: New Vegas) for years now. These not only still look spectacular on modern hardware, but entire new quest lines and regions are available via modifications that are easily managed via the (GPL'ed) mod management tool.
With the release of Civilization V: Brave New World DLC, Civ V has finally become a superior successor to all its predecessors (though it hasn't quite eclipsed the story orientation of Alpha Centauri), so that gets its fair share of game time, too.
Finally, I was bored the other day and decided to finally try out GTA:IV. While I had been a fan of the original two GTA games, the 3D once since hadn't managed to include sufficiently interesting story and characters to really engage me. So while I'd played the Mafia series, the recent GTA games were tried and quickly set aside. Imagine my surprise to discover that GTA:IV had actually improved. The characters are still lacking depth, but then so are those in Bethesda games. The important thing was they were finally engaging enough to not seem completely interchangeable, and the quests were aligned to some kind of plot I could follow. So for now, I'm happily playing through it and look forward to when GTA:V is released for the PC.