.NET Framework Software Development Kit
- The official .NET SDKs, redistributables and service packs from Microsoft.
- http://microsoft.com/netframework/

Mono project
- Provides the necessary software to develop and run .NET client and server applications on different platforms.
- http://www.mono-project.com

Shared Source CLI
- A shared source implementation of the .NET runtime, C# compiler, and JScript compiler from Microsoft.
- http://msdn.microsoft.com/library/default.asp?url=/downloads/list/netdevgeneral.asp

DotGNU
- An initiative to create an open source implementation of the .NET runtime and related development tools.
- http://www.gnu.org/projects/dotgnu/