NV open sourced CUDA in 2011, but I don't believe there are any other implementations out there. The rest of the world continues adopting OpenCL and now the whole Khronos supergroup is super hyper for Vulkan (NV even giving a solid thumbs up), with Apple and NV being the two rogue vendors pushing proprietary wares (Metal and CUDA). Even with NVidia doing really *really* well in the GPGPU market, even with a really great dev env, the extreme proprietary-ness of CUDA makes it really hard to sell to the alpha techies.
Cuda has a lot of traction in academic and applied fields, but the technical industry doesn't take it seriously, isn't comfortable saddling themselves to a one-trick-horse offering from NVidia. This ridiculously powerful box, and it's cool software with cool visibility into a neat problem, but it's really a pipeline play, to get you into NVidia's world. For some, going full in on NVidia is ok, but I don't think it's unlike going full in as a MS Developer or iOS developer- you're picking up, putting on the blinders, and all you'll be able to do is sprint towards a fixed, not too far away point.