Forgive me, I only like to play with computers and don't know the first thing about programming:
Mono is a cross platform [there are Windows builds], open source .NET development framework.
Mono, the open source development platform based on the .NET framework, allows developers to build Linux and cross-platform applications with improved developer productivity. Mono's .NET implementation is based on the ECMA standards for C# (http://www.ecma-international.org/publications/standards/Ecma-334.htm) and the Common Language Infrastructure (http://www.ecma-international.org/publications/standards/Ecma-335.htm).
three open source licenses:
The C# compiler is dual-licensed under the MIT/X11 license and the GNU General Public License (http://www.opensource.org/licenses/gpl-license.html) (GPL). The tools are released under the terms of the GNU General Public License (http://www.opensource.org/licenses/gpl-license.html) (GPL). The runtime libraries are under the GNU Library GPL 2.0 (http://www.gnu.org/copyleft/library.html#TOC1) (LGPL 2.0). The class libraries are released under the terms of the MIT X11 (http://www.opensource.org/licenses/mit-license.html) license.