DAIKATANA, of course! J/k, don't. It's only even available for sale for people to sate their curiosity about it, I think.
Anachronox: I'm still in my first play-through, and damn this is a good game.
Giants: Citizen Kabuto is a gem.
Fallout 3, Fallout: New Vegas, and Skyrim, for good open-ended sandboxes, with mods-o-plenty.
S.T.A.L.K.E.R. series.
Far Cry series.
Just Cause 2 is pretty good, IMO.
Serious Sam series, for plain old Doom-like blow up monsters action.
System Shock 1 and 2 (2 is easy to get from GoG or Steam, and you want one of those, for Win7 64-bit compatibility...I never could get my old copy working).
Baldur's Gate 1 and 2 for old school RPG action, along with Icewind Dale, Planescape: Torment, Fallout, and Fallout 2.
Dwarf Fortress, via Lazy Newb Pack, to get your geeky "fun!" on (kinda like Sim City, where the complexity will overwhelm you).