The SPACE Engine is a game engine based on SDL. SPACE is in an early development state. To get the source go to the Sourceforge page
Developed by Christoph Ehlen.
The SPACE Engine is try to create a 2D Engine with a good documentation.
I searched around the web but I was not able to finde a 2D engine i likes.
Well ClanLib is a realy good engine, but I dislike the complicate API.