Mine would have to be this game called spiral knights. You don't even need to download it if you can run it. The thing is that it's streaming from the internet.
Team Fortress 2. I bought it, but it's free now and honestly, it's worth more than the $60 games. It's got great class-based FPS gameplay, ****loads of content (and that's just the official content, there's community maps and modes too) and a cool item system.
