This weekend they are running a deal on Steam. For $10 you can buy Team Fortress 2 for your computer. I played this on Xbox Live in the bundle pack of the Orange Box and would have to say, $10 is definitely worth it if you have a good enough computer to play games on.
Just log in to your steam account and search for Team Fortress 2. Hurry up though, it’s only good for this weekend unless they extend it longer than planned.