Starter Kits XNA Creators Club
Innovative Games – Game Engine Tutorial
Gamedev Forum: Networking Engine for XNA
XNA Creators Club Forum: Physics Engines for XNA
Quickstart Game Engine for XNA
Ox Game Engine for XNA
Focused Games Framework
Silversprite: XNA -> Silverlight
http://roecode.wordpress.com/xna-gameengine-development-series/
http://www.ziggyware.com/weblinks.php?cat_id=10
http://www.gametheory.ch/index.jsp?positionId=64249