After 18 months of work, the Vulkan 1.0 graphics specification has been released [tomshardware.com]:
This is a little different from the Khronos Group's past launches, however, in that this time around there will be more than a specification PDF available – there are drivers, support documentation and a free SDK, and there is even a game that you can download with a Vulkan backend. There is a multitude of companies comprising the Khronos Group, and those in the working group for Vulkan include not only AMD, Nvidia, and Intel, but game engine makers and even Oculus VR.
[...] Unlike DirectX 12, Vulkan is completely open-source and royalty-free. Anyone who wants to use the code or adjust the code to fit their personal needs is free to do so, be that for private or commercial purposes. [...] DirectX 12 is Microsoft's graphics API, and it works only on Windows 10. Metal is an API made by Apple, and although it is also a low-overhead API, it works only on Apple devices. Vulkan, by contrast, works on many platforms. You can use in on operating systems as old as Windows XP on up to Windows 10, pretty much any Linux distro including SteamOS, and Android. Interestingly enough, Apple has opted not to integrate support for Vulkan into its devices -- although it is free to do so, so a day may come when Apple devices do have Vulkan support.
The development of the API owes a lot to AMD's Mantle [anandtech.com]:
Since Khronos's last major press update almost a year ago [anandtech.com] in March of 2015, not a great deal has changed on the technical side from a high level. After being gifted Mantle 1.0 from AMD – an action that significantly sped up the development process and bypassed the need to figure out some fundamental questions about how the API should be designed – the consortium went about adapting Mantle to serve as a wider, more generic API suitable for hardware from multiple vendors across multiple OSes.
The end result is that Vulkan has its roots firmly in Mantle, through Khronos has worked to make it very clear that multiple vendors are responsible for contributing IP that ultimately went into Vulkan. And while the specific low-level details of the API are beyond the scope of this article, I do know that the shader resource binding system is significantly different from Mantle, and that's not the only system that was updated or overhauled during Vulkan's development.
More coverage at Ars [arstechnica.com] and The Register [theregister.co.uk]. Check out Khronos Group's hub for the Vulkan 1.0 specification [khronos.org]. Both AMD [gpuopen.com] and Nvidia [nvidia.com] have released Vulkan drivers. Finally, here is the Valve-funded LunarG Vulkan-based SDK [lunarg.com].