EmuNations

Nin is a Nintendo Entertainement System / Famicom emulator that is in the early stages but is actively being developped. It's not very polished on the UX side, but can run many games without issues and has gamepad support.

PCem is an open-source, low-level PC emulator developed for Windows and Linux. It focuses on PC hardware from the 1980s and 1990s and has a variety of operating systems that can be installed, which can then be used to launch programs...

The Mesa project began as an open-source implementation of the OpenGL specification. Over the years the project has grown to implement more graphics APIs, including OpenGL ES (versions 1, 2, 3), OpenCL, OpenMAX, VDPAU, VA API, XvMC and Vulkan...