Galactic Civilizations 1 and/or 2 if you're into the 4x type of games. Customer support is pretty good, and the modding community is very active.
Operation Flashpoint, as mentioned above, is a superb game. Quite realistic too, as far as I know. There is also an editor included in the game, as well as two expansions which keeps replay values up. Just don't complain about the outdated graphics, the game is getting old. A sequel is in the works, though, and has been for some time, but I'm sceptic about a release. Then again, maybe Bohemia just doesn't want you to build up an unachievable imagination for the game pre-release fueled by hype? *cough* RAI *cough*
Halo, I thought was a damn fine game. I loved the story, but I attribute that mostly to the movie Aliens. It's sequel, on the other hand, is a piece of shit, and only available for consoles *pukes* and Vista (soon to be). I don't recommend picking it up unless you intend on playing Halo 3 (who's teaser is actually growing on me) which probably won't be availabe for the PC until Vista Millenium XXXP or some such bullshit.
Warhammer 40,000: Dawn of War is a classic already, and I love it dearly. This game is one of the finest RTS games I've ever played. There is an expansion pack (Winter Assualt) for it available and another one (Dark Crusade) in the works.
Then there's the moldy oldies that everyone loves, which most of them you can find on abandonware sites;
X-Com: UFO Defense (a.k.a. Enemy Unkown)
Squad Leader by Microprose is one of my favorite WW2 TBS games.
Beneath A Steel Sky (I'm not done with it yet, however)
Escape From Monkey Island
Speedball
Final Fantasy VII (best Action-Adventure "RPG" I've ever played)