Simple Directmedia Layer (SDL) is the gold standard for low-level, cross-platform game development. It is an open source library that is used at the layer where one might use DirectX, so it's useful both for from-scratch development and for powering game engines. It provides abstractions for video, audio, input, and various system-provided functionality. In many cases, if you're building a cross-platform game, SDL is extremely good at helping you delete a large number of #ifdefs in your project...and entire directories full of code.
SDL is used in the Steam Client, is crucial to the Linux game development ecosystem, and has been used for various platforms in every generation of the Unreal Engine, idTech, Source Engine, etc.
A game written to use SDL might work with almost no code changes across all major desktop operating systems, mobile phones, web browsers, and even the Raspberry Pi. Nintendo shipped SDL in over 7.5 million NES and SNES Classic devices. There are SDL-based games