#develop is a free Integrated Development Environment (IDE) for C#, VB.NET, Boo, IronPython, IronRuby and F# projects on Microsoft's .NET platform. It is written (almost) entirely in C#, and comes with features you would expect in an IDE plus a few more.
The #develop project started on September 11th, 2000.
http://www.icsharpcode.net/opensource/sd/