Marathon, from when Bungie made good games. Saw a guy playing it in the computer lab. Had to buy a new PowerMac to run it - I mean to do homework. I loved it because it wasn't linear like other games of that era. You could shoot an alien, play for an hour, and go back and find its body still lying where you killed it. It also had a good map, which is missing from lots of high-end shooters today. That was from a game that came on four floppies and ran on 4mb of ram!