Though I'm the last to report it there maybe a couple of people who have not heard of this.

Flat Red Ball is a 2.5d game engine which has been around in the Managed DirectX world for at least a couple of years now. They have just released a full conversion to the XNA framework meaning you can write games for Windows and Xbox 360. The API is not compatible with the old version but the new version is much more .Net like and includes all the years of experience of the old version. The author keeps this engine very well maintained, often with monthly updates so you know you are getting something that won't go away.