SmallBASIC is a fast and easy to learn BASIC language interpreter
CGL (C Game Library) is a multipurpose library
An implementation of OpenGL 3.x-ish in clean C
BASIC for the modern era
The penultimate portable graphics library
Low-Level Game Engine for Retro Platforms
A kit for cross-platform & embedded development in modular languages.
A C Compiler/Editor for Retro 6809 Machines (Colour Computer/Dragon)