Defcon - If you have Steam, and don't mind semi-plain graphics, this game is fun. If you've ever seen the movie WarGames, you'll know what this is about.
Age Of Mythology - I prefer it to AoE, just because of the mythical element.
Starcraft - Which has already been denied, I guess.
Also, there's a mod for Source that recreates Goldeneye. It's pretty decent, and does away with the N64's clunky interface.
PS. Open14 is going on my list.