With the Vulkan API still in flux and not finalized until later this year, Valve has been developing their own Intel GPU reference driver for Vulkan to help early adopters boot-strap their code. Valve said during their presentation today at GDC2015 that this Intel Linux driver will be open-sourced, but they haven't provided a time-frame for doing so.
Valve also confirmed that the Source 2 Engine supports the alpha Vulkan API today and that Vulkan will be supported across the board on Steam Machines.
While Intel graphics hardware isn't the fastest, it's very common and also most easy to target for an open-source driver given Intel's extensive hardware specifications / programming documentation that they routinely release for new generations. The Intel DRM kernel driver is also in great shape. On the NVIDIA side, the Nouveau community is largely left for reverse-engineering. For open-source Radeon graphics, AMD mostly has just been putting out code these days with not as much key documentation as in recent years, and when docs do come it is after the fact.
A Vulkan Intel Linux graphics driver used by game developers sounds very promising for having good support for this new API and most likely high-performance was a priority in the development of this driver by Valve and likely their partners at LunarG. In my most recent Intel Broadwell Windows vs. Linux benchmarks, Intel's Windows 8.1 was still slightly faster over the open-source Linux driver.
I'm working to find out more information on this new Intel Linux driver -- and if it was Mesa-based or not, so stay tuned to Phoronix for updates. All around though this should be great news for Intel Linux gamers and those wishing to experiment early with Vulkan while Intel's official Mesa driver still lacks OpenGL 4.0 compliance.