As promised, Valve has made its SteamOS available to all — and barring server issues, you can download it right now. The Linux-based operating system weighs in at 960MB, and will be used to power the company's Steam Machines game platform. Today Valve is also shipping out prototype units to 300 lucky testers. The free download is only really for the most dedicated among us, however. "Unless you're an intrepid Linux hacker already," Valve says, "we're going to recommend that you wait until later in 2014 to try it out." If you're feeling brave, you can grab it at the source link below — though of course you'll have to wait to play around with the fancy new controller.
Update: Valve has published a SteamOS FAQ, including installation instructions. Note that Valve says installation will erase everything else on your machine. According to Valve, SteamOS is a fork of Debian Linux, with the first version based on Debian 7.1. Presently, it only works with Nvidia graphics cards, as AMD and Intel graphics drivers are still a work in progress, though you can run the OS with your choice of Intel or AMD processors. The OS is designed to automatically update itself from the latest repositories.
Update 2: Valve has offered a new download site.
Update 4: Valve now has a friendly user-facing website explaining just about everything.