OpenCL
OpenCL predates CUDA's dominance and was, for years, the only serious vendor-neutral answer to "how do I write one program that runs on GPUs from multiple vendors, plus CPUs, plus other accelerators." It still runs today, still ships in every major GPU driver, and still matters in a specific set of niches — but it lost the mindshare battle for general-purpose GPU compute, and understanding why is as useful as understanding the API itself.