Engine Nine is the most popular open source XNA game engine that allows you to create amazing 2D and 3D games in C#. It runs on Windows, Xbox, Windows Phone and Silverlight. Licensed under the unrestrictive MS-PL, it is completely free to use with your commercial
or non-commercial applications.
PM> Install-Package Nine
To get started, read the tutorials and documentations. See what other people have said about Engine Nine:

“Engine Nine is breathtaking” –Nelxon

“You are doing some very ingenious architecture work” –TombstonedGuy
Features
- Platforms
- Runs natively on Windows, Zune, Xbox 360, and Windows Phone 7.
- Runs natively on web browser using Silverlight 5 (Work in Progress).
- Can be ported to iOS, android though Mono (Future).
- XAML based content authoring.
- Graphics
- 2D sprite, sprite animations and text rendering.
- Support both forward & deferred lighting
- Support directional light, point light and spot light.
- Support shadow mapping and normal mapping.
- Build-time shader linkage from fragments.
- Terrain splatting and level of details.
- Flexible particle system effects and soft particles.
- Basic primitives: box, sphere, teapot, axis, arrow, point, line, grid.
- Extensible post processing framework.
- Built-in high dynamic range, bloom effect, depth of field and blur effect.
- Animations
- Skeleton animation, animation blending and custom animation controllers.
- Full support for Xbox Live Avatar.
- Transition system for creating smoothed tweening.
- Physics
Donate
Engine Nine is free to download, use and modify. If you find it useful, you can
donate via PayPal or:

Thanks for the donations from:
- Tobias Lawrenz
- Jan Kondelík