Portal is now available as a Beta. This beta adds Linux support and converts the game to the new Steam Content delivery system.
This beta involves significant changes for all platforms and your testing is appreciated, please report any bugs to our GitHub page, https://github.com/ValveSoftware/source-1-games/issues.
Linux users can simply install Portal to access the beta. For Windows and OS X users right click the game in your Library, choose properties and then go to the Beta tab. Select the SteamPipe beta to start testing. Under Windows and OS X to opt out of testing simply deselect the beta option on this same page.