Spacewar is the first fully-interactive PC game made.
It was remade on Steam, and now you can get it for free.
Simple enough, click on this link in a chat window, so try pasting it in someone who already has it's chat.
It's very fun, you should all try it. :3
Truce's quote, for probably better instructions..
Go to steam://install/480 in your Steam Browser to get a free game called Spacewar. It's some pixel-based triangle-deathmatch near a blackhole. Doesn't work on XP, apparently, and I can't find what port to forward to host for it.
I'm hosting a game, gogogo.