Valve is working on a lot of things besides games, and one of their most esteemed projects is the Source SDK, which has received support for VR mode on Linux.
Implementing virtual reality support in the Source SDK means that a lot more developers who are using Valve's engine will be able to implement the VR technology into their games.
Highlights of the latest Source SDK:
• The readability of the UI in VR has been improved;
• The IPD calibration tool has been removed. According to the developers, TF2 will now obey the Oculus configuration file. You can use the Oculus calibration tool in your SDK or install and run "OpenVR" under Tools in Steam to calibrate your IPD;
• A dropdown menu has been added to enable VR mode in the Video options and the -vr command line option has been removed;
• The ability to switch in and out of VR mode without quitting the game has been added;
• Support for VR mode on Linux has been added.
Check the Github changelog for more details about the changes implemented in Source SDK.