Available on Steam

Steam is a distribution service and not publisher, thus game being available there does not mean it was published by Valve (but a game being published Valve would extremely likely mean it's available on Steam). Nor does availability on Steam mean they use the Steamworks API or anything else either.

Editor note: do NOT remove reference links to Steam that redirect to the Steam's front page, these games either are not sold where You are or the games are not sold anymore at all. However, they still exist on Steam in a way that they can still be installed and played by those who bought them when they still were available there normally. And the associated ID numbers point to the same thing still.

Steam originally only officially supported Ubuntu Linux, even though the client no longer blocks installation on other distros and the games may work out-of-the-box on other distros. SteamOS was to be based on Ubuntu but Valve later decided to use Debian. It seems SteamOS may not actually maintain strict Debian compatibility however. Valve continues to support Ubuntu, therefore all 'Steam powered' Linux games should be tagged with Ubuntu (for now). Developers are free to name other distros as well and should be tagged as such when applicable.

A game being in Steam Greenlight is NOT valid reason to add this tag to them, as Greenlight is not guaranteed process to get to Steam. Greenlight is also not the Steam distribution service itself nor the webshop, but rather a service to get a game there with simply popularity voting. So, to repeat, do not tag games with this if they're Only in Greenlight.

See also: Steamworks API, Valve Anti-Cheat, Steam Achievements