I have one I'd like to recommend but I'll save it for last. If anyone else puts it in first then I wont.
Oh boy, where do I begin? The Stronghold series, Rise of Nations and it's expansion, Half-Life if you haven 't played it, Unreal Tournament (all of them) and Call of Duty are all good games.
Diablo 2:LOD--The best fucking game ever. If your looking for something of lesser value though, any blizzard game. They've made some of the best games I've ever played. Really, those guys can make a good fucking video game. PS: Solitaire
Can you specify if it's downloadable, and if you need a cube or can play it in the pc? Any of the above.
So far every game mentioned is for the PC. You can buy them legally or download them on Kazaa or something. Just don't say I never warned you about viruses.... If it's a free game you want, try Survival Project, that's pretty fun.
I have to recommend everyone Second Life; it's like hippyland. You can do pretty much anything(fly, build houses, ships,cars, design every part of your avatar, take photos, dance, teleport, go to meetings) but mostly explore and learn freely or sit on a bridge watching the sun go down. It's almost like a trip. http://secondlife.com/