https://github.com/baldurk/visor Vulkan Software Renderer
https://github.com/Chabloom/VulkanOnD3D12 Vulkan to Direct3D 12
https://github.com/msiglreith/rostkatze Vulkan to Direct3D 12
https://github.com/gfx-rs/portability Vulkan to Direct3D 12 and Metal and also to Vulkan itself again
https://github.com/KhronosGroup/MoltenVK Vulkan to Metal
https://moltengl.com/moltengl/ OpenGL ES to Metal
https://github.com/g-truc/glo OpenGL to Vulkan
https://github.com/disks86/VK9 Direct3D 9 to Vulkan
https://github.com/doitsujin/dxvk Direct3D 11 to Vulkan
https://github.com/Microsoft/angle OpenGL ES to Direct3D
http://www.zeus-software.com/downloads/nglide Glide to Direct3D and Vulkan
more coming ...