EZ2D - A 2D Game Engine
EZ2D is an easy to use, game engine targeted for new game developers. Of course as its title implies, it uses 2 dimensional graphics. It uses SDL as its main API (sorry DirectX users), but very little knowledge of SDL is actually required.
https://ez2d.sourceforge.net/
Simple DirectMedia Layer
Computers, Programming, Games, Libraries - Simple DirectMedia Layer. A cross-platform multimedia library designed to provide fast access to the graphics framebuffer and audio device.
http://www.libsdl.org/
LibKSD - Welcome and News
Computers, Programming, Games, Libraries - libksd. A cross-platform game framework (written in C++) that would allow you write your game once and compile it for multiple operating systems and environments, including: Linux/X11, Linux/SVGAlib, MacOs, Windows, and BeOs.
https://libksd.sourceforge.net/
The Grand Unified Toolkit (GUT)
Computers, Programming, Games, Libraries - The Grand Unified Toolkit (GUT). A cross-platform C++ API for the development of games and other forms of interactive media.
https://gut.sourceforge.net/
Regimental Command
Computers, Programming, Games, Libraries - Regimental Command. A generic armored combat system.
https://regcom.sourceforge.net/
Titan
Computers, Programming, Games, Libraries - Titan. A C++ based library to load/save game-related file formats, such as graphics, sound and 3d meshes.
https://titan.sourceforge.net/
Jet3D Project
Computers, Programming, Games, Libraries - Jet3D. A (commercial) free and open source realtime 3D graphics engine and SDK.
https://jetpp.sourceforge.net/
Overview - ClanLib SDK
Computers, Programming, Games, Libraries - ClanLib. A multi-platform game development library.
https://clanlib.org/