SynD
The source to a (very) old engine of mine. SynD was created specifically for side-scrolling platform games. The source is a little messy in places, and mostly C, but someone may enjoy tinkering with it.
GMF Compiler source v0.48
This is the full source code to the VB version of the GMF Compiler. It is pretty well commented and shouldn't be too hard to understand. Please make sure you read the readme.txt and license.txt before opening and using the source code.