Most likely ubuntu as that seems to be their defacto test platform. If you download steam from their site it comes packaged in a .deb format. They have pretty s

Most likely ubuntu as that seems to be their defacto test platform. If you download steam from their site it comes packaged in a .deb format. They have pretty significant investments in that stack, and even list it as their favorite flavor: "Not running Linux yet? Ubuntu is our favorite version of Linux." (at the bottom of this page: http://store.steampowered.com/about/ http://store.steampowered.com/about/)