Was willst du machen? Anwendungen, Spiele, etc?

Für Anwendungen könntest du mit FLTK viel Spaß haben... Es ist IMO anständig zu benutzen, plattformunabhängig (OpenGL-basiert) und allgemein lustig.

Ansonsten liste ich einfach mal das Ergebnis einer schnellen Suche bei SourceForge auf (ein paar interessante Projekte):
Kyra Sprite Engine
Kyra is a simple, fully featured, industrial strength Sprite engine written in C++. The Kyra engine is suited to 2D, isometric, and quasi-3D games. It is built on top of SDL for cross platform use.

EclipseEngine Eclipse is a 2D game engine based upon SDL and FMOD to allow the greatest freedom acoss multiple platfoms. Designed to be easy to use Eclipse will take care of the mundane tasks and let you get on with the most exciting part - creating your game!

ZEngine - 2D SDL and OpenGL Engine
ZEngine is designed to provide a powerful yet easy to use 2D game API using OpenGL for fast 2D drawing and SDL for everything else, it is completely cross-platform and the class based design makes it easy to learn and use.

EZ2D Game Engine
EZ2D is a 2-Dimensional Game Engine based off of SDL. Using SDL_Mixer, SDL_Image, and the main SDL libraries, this engine provides an easy to use interface for Graphics, sound and input.

PORTALEngine
PORTALEngine is a Game Engine for 2D RPGs that will have a mixture of features from Japanese-style RPGs and Western-Style RPGs.
PORTALEngine es un motor para hacer RPGs 2D con una mezcla de caracteristicas de RPGs japoneses y RPGs Occidentales.