Call of Combat is a real-time, tactical multiplayer war game set in WWII. Each player is in control of four soldiers, as a part of a larger team, controlled by other human players. Call of Combat is a clone based off a 2am game called "Chain of Command".
This game is free to play (always has and always will be). I'm not affiliated with it, but I do play it. I've heard that linux players have got this to work, but I haven't yet done it myself. The game is written in java.
If anyone gets it to work on their machine, please let me know. This would be one more reason to use linux for games.