Engines, Libraries, and APIs

Tools for those that don't want to reinvent the wheel.

megumeme

Open Source
Closed Source

Open Source

assimp: Model loading library made with C++.
Bevy: Popular Rust game engine.
bgfx: Thicc C/C++ rendering library with numerous bindings for other languages.
Dear ImGui: C++ GUI library.
EnTT: Fast entity component system written in C++.
egui: Rust GUI library inspired by Dear ImGui.
GLFW: Multi-platform C library for making windows and handling input events.
Godot : 2D/3D game engine written in C++.
libGDX: Java framework with bits of C/C++ for performance.
LÖVE: 2D game engine in Lua.
Lua: Lua is a simple and powerful language that can be used to embed scripts in programs.
miniaudio: A simple cross-platform audio library written in C, contained in a single source file.
MonoGame: C# framework for making games and engines.
Ogre: 3D game engine made in C++.
OpenGL: Ubiquitous cross-platform rendering API.
OpenGL Mathematics (GLM): glm is a ubiquitous math library made for use with OpenGL. It is also commonly used with other graphics API such as Vulkan.
PhysX: A powerful C++ physics library.
pygame: Python library for game dev.
Ren'Py: Engine for making ~~porn~~ visual novels.
raylib: Simple to use C library. The raylib website provides examples and simple games.
SDL: C/C++ library that provides access to graphics, audio, and hardware.
spdlog: Speedy C++ logging library.
sokol: Header-only C library that provides a large number of abstractions and utilities. This includes a wrapper for OpenGL + Metal + D3D11. There are also some handy samples to look at.
Stride: C# engine under the MIT license.
Vulkan: OpenGL's hot younger sister.
wgpu: Rust graphics library based on the WebGPU API.

Closed Source

The term "closed" also includes "pay to view" code and code with strings attached.

Havok: Physics engine, also has an AI module (pathfinding, navigation meshes, etc.).
RPG Maker: Tool to easily make Niponese text and number simulators. But more likely you will use it to make a porn/fetish game. Allows for customization if you know Javascript.
Unity : ~~Widely known, uses C#.~~ OH NO NO NOOOOOOOOO
Unreal: AAA game engine, uses C++.