As of 2016, CUDA is the top cat.
http://stackoverflow.com/questions/7263193/opencl-performance-vs-openmp
Directive based: you write stuff like:
http://stackoverflow.com/questions/19494786/openacc-vs-openmp-mpi-differences
/* Fork a team of threads giving them their own copies of variables */
#pragma omp parallel private(nthreads, tid)
{
/* Obtain thread number */
tid = omp_get_thread_num();
instead of compiling a kernel.
https://en.wikipedia.org/wiki/OpenACC
NVIDIA's OpenMP, similar to OpenMP: http://stackoverflow.com/questions/19494786/openacc-vs-openmp-mpi-differences
Google's choice for Android: http://stackoverflow.com/questions/14385843/why-did-google-choose-renderscript-instead-of-opencl
Google somewhat opposes OpenCL, maybe because it was created by Apple?
https://en.wikipedia.org/wiki/Metal_%28API%29
Apple's response to Google's RenderScript.
Microsoft, Windows, Xbox.
https://en.wikipedia.org/wiki/Cilk
Intel's
https://en.wikipedia.org/wiki/DirectCompute
Microsoft's
https://en.wikipedia.org/wiki/Unified_Parallel_C
OpenGL compute shaders http://stackoverflow.com/questions/15868498/what-is-the-difference-between-opencl-and-opengls-compute-shader
Easier to integrate with graphics stuff.