Not Steam, but free: My favorite old game is Bungie's Marathon series (which consists of the official trilogy and many third party scenarios, maps and levels [they were fun to create and play]). You are a security officer aboard the UESC Marathon colony ship. Are you the 10th Mjolnir Mark IV cyborg? You may well get answers to that question as you battle your way through the Marathon; alien ships, planet and moon. This FPS works on current Windows, Mac and Linux machines by using the Aleph One engine. The open source engine and the games are free, thanks to Bungie. You may get them here: http://marathon.sourceforge.net The site has a few pics. Here's one from Bungie: The graphics (resolution, etc.) have been improved in Aleph One over this old pic.