EARTH 2150 is old school and really fun. Good luck finding a legal copy though.. It seems the game has turned into a sort of collector's item or something or other... I was told anyways... But if you are searching for a torrent, I can help you out with that!

Its cool, one of the first full 3D RTSs... The point of the game is to annihilate your foes and steal their resources, before they do that to you... and if you dont... well, expect things to get "hot."
Rise of Nations is another good game, but its cartooney graphics get old for me. Other than that, though it is a fun game. It is really fun to build 37 nuke silos and then launch em all at once... not only do you litterally obliterate everything on the map, but you cause the game to "Armegeddon" which makes it so EVERYONE loses.
Dungeon Keeper 1 & 2 are ridiculously fun. Especially the first. I really can't do the game justice with words. Just play the damn game and enjoy!

Be wary of the second one though, apparently windows XP doesn't like it, and it is damn near impossible to get the renderer to work on an XP (biggest reason I am upgrading to 7!)
Anything from the Total War series is also good. Personally, I am an original medieval fan (I didn't like the look of the second). It is fun to take over all of europe. Be warned: This game is TIME CONSUMING. You thought stalker was bad for sucking up hours... try beating Medieval Total War in one sitting: ain't happening!
Hope I helped! Cheers!