Most people here seem to know about simian's Mono but not about the Dot GNU project.

symian's mono project http://www.go-mono.com/ is developing an open Source implememntaion of the ECMA CLI for Unix. The CLI is released under LGPL and the C# compiler under GPL License.

GPL License: http://www.opensource.org/licenses/gpl-license.html
LGPL License: http://www.opensource.org/licenses/lgpl-license.html

The class libraries are under the MIT license: http://www.opensource.org/licenses/mit-license.html

Here is quick info on the licenses:

GPL : If you are changing the source code, then you must make the changed source code available free to others for them to change and distribute. You are not supposed to use Non-GPL ed code in your project.

LGPL: Same as GPL but linking to non GPLed Libraries is allowed.

MIT: You can use the source code to make your own derivative work without releasing your modified code.



Other Open Source CLI project

Portable.NET
http://www.southern-storm.com.au/portable_net.html

This project comes under the Dot GNU project ( much more bigger project that deals with many aspects of web services ).

All the components (CLI, Class libraries, C# compiler) are under the GPL license.