The Cyclone Game Engine is a cross-platform 2D/3D game engine built upon MonoGame and the XNA Framework to help make it easier and save time creating games. The engine is still in its infancy stages of development. It is primarily built to serve the needs of Steel Cyclone Studios' game projects. More updates are coming so stay tuned!!!
JigLibX, which stands for Jiggle Library X, is a C# port of the physics engine "JigLib" specially designed for XNA. I have upgraded the standard XNA 3.1 version of JigLibX physics library to XNA 4.0. It is available to download here.