.NET Framework Software Development Kit: The official .NET SDKs, redistributables and service packs from Microsoft.
DotGNU: An initiative to create an open source implementation of the .NET runtime and related development tools.
DotGNU Portable.NET: A project (part of the DotGNU) to build a suite of free software tools to build and execute .NET applications, including a C# compiler, assembler, disassembler, and runtime engine. The initial target platform is GNU/Linux, with other platforms to follow in the future.
Mono Project: An open development initiative sponsored by Ximian that is working to develop an open source, Unix version of the Microsoft .NET development platform.
Shared Source CLI: A shared source implementation of the .NET runtime, C# compiler, and JScript compiler from Microsoft.