Vulkan
Modern low-level graphics + compute API from Khronos, 2016. Successor to OpenGL. More control, less overhead, better suited to modern multi-core CPUs.
Built on AMD's Mantle (2013, donated to Khronos). Cross-platform: Windows, Linux, Android, BSD; Apple runs it via MoltenVK (Vulkan → Metal). Used by: Doom Eternal, Red Dead Redemption 2, DXVK (DirectX → Vulkan, powers Steam Play/Proton). More complex than OpenGL — explicit synchronization, descriptor sets, render passes. Vulkan 1.3 (2022) standardized dynamic rendering. Vulkan Compute competes with CUDA/OpenCL in some cases.