Back in 2003 Valve launched their Steam content delivery service, since then the service has gone on and on and is now utilized by 25 million gamers worldwide, also there are apparently over 1,100 games available through Steam.
As you can imagine the success of Steam has brought on a lot of interested from platforms other than Microsoft Windows, Valve recently revealed that they are currently developing a Mac OS X version, this will no doubt open-up Mac OS X for gamers.
Phonorix has recently discovered unreleased Steam Linux binaries, these are apparently being actively developed and a screenshot shown on SlashGear (link towards the end of this post) appears to show Steam running on Ubuntu.
There have been no official comments made by Valve yet, however I am sure once the Mac OS X version is launched more light will be shed on a Linux version.
Also See: Steam Store down for hours say gamers