One small step for Stardock, and a huge leap for API.
DirectX 12 is the new API exclusive to Windows 10. It's the most recent revamp of DirectX, which is what allows Windows to perform well with gaming. The new version is faster and more efficient, which opens up a world of potential for gaming graphics.
Until now, however, no games had been built on DirectX 12. Stardock changed that by releasing Ashes of the Singularity on Steam Early Access this morning for $40. Though be warned: Stardock says that the pre-beta "requires very high-end hardware and should only be joined by people with susbtantial technical expertise and experience in real-time strategy games."
However, don't be too worried on missing out. Nvidia has said Ashes of the Singularity is not a solid indicator of DX 12 performance. But at least someone is taking advantage of DirectX 12.