I'll try to post stuff that's similar to the games you already listed.
If you like Real-Time Strategy, I suggest getting Starcraft 2, once it comes out. If you're into first-person shooters, Call of Duty 4 is a must-have. It is, in many people's opinion, even better than Halo 3.
Personally, I am a big fan of games that focus on Player Verses Player combat. Team Fortress 2 is an excellent team-based first-person shooter, and includes many unique, humorous characters.
If you like fantasy-based PvP, I highly recommend buying Guild Wars. After buying the game itself, Guild Wars is free to play indefinitely, although you *can* purchase any or all of the expansion packs at any time. Although characters gain experience and grow levels, the level cap is relatively low, and as a result, the game is not a Runescape-type ordeal where you have to play 10 hours a day just to get anywhere, but players actually have to use strategy to win.
If you like futuristic RPGs, you might want to look into Anarchy Online. The game does have an option to pay for extra features, but the basic version is free indefinitely and is a sizable game by itself. I warn you, though, the setup file is a 1.5GB download, and the game itself takes up much more space than that when installed.