Beginning this evening, if you fire up your Steam Linux client, Valve's Portal and Portal 2 games should now be present for Linux gamers. Portal was released in 2007 while Portal 2 came in 2011. The Portal series is a very popular first-person puzzle-platform games out of Valve with a large emphasis on teleporting and dealing with various objects.
These latest Valve game releases for Linux is very good news at a time when Steam Linux usage is declining, albeit many gamers are awaiting Left 4 Dead 2 and Counter-Strike: GO on Linux.
For those that didn't previously purchase Portal 2 for Windows or OS X, the game can be purchased via Steam Powered for $19.99 USD.
Update: It appears Portal 2 isn't publicly available to all via the Steam Linux client, but expect it soon.