The Farseer Physics Engine is an easy to use 2D physics engine designed specifically for Microsoft's XNA platform. The Farseer Physics Engine focuses on simplicity, useful features, and enabling the creation of fun, dynamic games.

Mercury Particle Engine is a 2D particle system for Microsofts XNA Framework. It allows developers to add impressive visual effects to their Windows & XBox360 games with minimum effort.

Charles Humphrey's Randomchaos 3D Engine project.

XNA Paralax Engine is a 2d Paralax scrolling component. This project is designed to enable developers of XNA projects to easily use and create games that have scrolling backgrounds without the need to develop complex scrolling management systems.

The QuickStart Engine is an engine designed to allow a programmer to get a 3D game started in XNA Game Studio as quick as possible. Still in prototype phase, this project is designed as a moderated community supported project. QuickStart is in a constant state of evolution, with new versions being released weekly or sooner. If you feel something is missing, simply add the suggestion in the discussion section and there is a good chance you may see it soon, if it is feasable. You're also welcome to join in an help develop for it.

DPSF - Dynamic Particle System Framework. Create custom particle systems in XNA quickly and easily.