Well the long rumoured Steam for Linux client has now been realeased. So you can now shoot your friends while wearing a silly hat on Linux too.

Seriously though this is a good thing for the Linux platform - it's breaking it into a new market, and a sign that it's being taken seriously and as a mature platform by an important company (given that not only do Valve make very popular games they also have a dominant position in the gaming digital distribution market). If I were Microsoft though I'd be worried that one of my traditional markets is being eroded - Steam for Mac, Origin for Mac, Steam for Linux, ubiquitous console ownsership... Heck even Microsoft themselves are canabilising their own Windows games in favour of the xbox.