I'm not a gamer, not even close to it. But today and yesterday I have been playing Armagetron Advanced a lot. It's a simple 3D open source game that runs on Windows, Linux and the Mac (yay!) based on the Tron movie. What I like about it that it's live action and in essence very simple. You can play quite well by simply using two keys (one to go to the left and one to the right). You can play it locally or on the internet. I just won my first game online, wooh! ;)
You can give it a try for free here: Armagetron Advanced website. Recommended!