DEFCON: everybody dies. It's Windows only currently, but Linux and Mac versions are in preparation. There's a demo, but the full game is cheap enough to count as almost free.
This one is derived from "War Games". It's Global Thermonuclear War, presented in the cold blodded view like on the control monitors in your favorite imaginary command bunker. You command a sea fleet of submarines to blow up other ships or launch nukes, carriers to launch airplanes or hunt submarines, destroyers to shoot at other ships and airplanes (not submarines, note that in order to be interesting, realism is not part of the game). Airplanes are bombers with nukes and conventional bombs against ships or fighters for scouting and killing defenseless bombers. You see, rock-paper-scisors all the way
In the beginning, you also place stationary airfields, radar stations and, of course, missle silos. Another wildly unrealistic, but fun, twist is that silos can either be in air defense mode where they shoot down planes and incoming nukes or in nuke launch mode, and every mode switch takes enough time that you're pretty much defenseless while you launch from your silos. It's a neat game if you see it as an abstract thing, and deeply disturbing if you let the scenario get to you.There are some catches. First, if you're playing to win and not to watch your navy ripped to shreds and your cities leveled to the ground, you have to micromanage a lot. You need to time your nuke launches, often more than 40 almost at once, to hit the targets almost simultaneously to overwhelm the air defense. You need to keep a constant eye on your fighters, they have very little fuel and tend to crash if you don't manually order them to land. You need to keep your bombers out of the range of the destroyers.
It's a bit hard to find partners to play with: EVERY game appears on their master server list, even local ones where someone just wants to test the AI. There are no dedicated servers, so the server runs on some random player's machine, and without port forwarding, that doesn't work, so you get tons of timeouts. And when you finally found a game and start playing, the other guy quits when you've send his navy to the bottom of the sea and melted his silos, and of course if the quitter ran the server, it's game over. Happened on all three games against real people I've had so far. Grr. So, if anyone from here want's to play against me, I'll be on our IRC channel (#armagetron on freenode) when I'm looking for a partner. In DEFCON, I'm the "Covered Duck"
