BizHawk is a multi-system emulator written in C#
CGL (C Game Library) is a multipurpose library
Resilient Image Library - successor to DevIL
Homebrew save management, editing tool and memory trainer for Horizon
A single-header ANSI C gui library
Access to Irrlicht Engine SDK from assembler, C, Python, Basic, Pascal
A C++ library for programmers transitioning from "Game Maker Language"