FreeImage is a library project for developers who would like to support popular graphics image formats (PNG, JPEG, TIFF, BMP and others). Some highlights are: extremely simple in use, not limited to the local PC (unique FreeImageIO) and Plugin driven!
KAGEfx is a framework to load shader programs based on the OpenGL Shading Language contained within an XML file that holds descriptive metadata about the shader and to replace shader modules on the fly with respect to their level-of-detail metadata.
Read, export and render CMV (Corel MOVE!(tm) 5.0) files.
Provides ability for cross-platform reading, exporting and rendering of CMV (Corel MOVE!(tm) 5.0) files. Includes Windows player with ability for playing of CMV animation and extracting embedded resources.
JUCE is a C++ toolkit for building cross-platform applications on PC/Mac/Linux/iOS/Android. Particularly good for complex, customised GUIs and audio/midi processing, it also includes a vast range of classes to help with all your day-to-day programming tasks.
The Objective-C Interactive Simulation Abstraction Layer
SimKit(because GameKit was taken) is designed to allow quick and easy cross platform abstraction away from platform dependant drawing and physics libraries. The APIs provide a generic way to make common calls for drawing, animating, and otherwise manipulating 3D entities, as well as shading and texture information.
Sonora is a port of the PopCap Framework to Mac OS X. Sonora is an OpenGL and SDL based game engine that includes everything a developer needs to quickly create 2D games.
gnoga - The GNU Omnificent GUI for Ada
Gnoga uses modern web technologies to allow simple creation of cross platform GUIs for Ada with native or custom look and feels that perform on par to native toolsets locally and can be easily and securely remoted as web apps over the internet as well.