Sur plateforme Microsoft : Sharp Develop
http://www.icsharpcode.net/OpenSource/SD/
The Open Source Development Environment for .NET
#develop (short for SharpDevelop) is a free IDE for C#, VB.NET and Boo projects on Microsoft's .NET platform. It is open-source, and you can download both sourcecode and executables from this site. In addition, you can find the latest information and changes on #develop, as well as get in touch with the team in the forum.
Sur plateformes Linux, FreeBSB, Mac OS X, Microsoft : Mono
http://www.mono-project.com/Main_Page
Mono 2.4 has been released! The Mono Project aims to make developers productive and happy: Mono 2.4 is our gift to the world. Sponsored by Novell (http://www.novell.com), the Mono open source project has an active and enthusiastic contributing community and is positioned to become the leading choice for development of Linux applications.
Multi-Platform
Runs on Linux, OS X, BSD, and Microsoft Windows, including x86, x86-64, ARM, s390, PowerPC and much more
Multi-Language
Develop in C# 3.0 (including LINQ), VB 8, Java, Python, Ruby (http://www.ironruby.net/), Eiffel (http://www.eiffel.com/), F# (http://research.microsoft.com/fsharp/), Oxygene (http://remobjects.com/oxygene), and more
Binary Compatible
Built on an implementation of the ECMA's Common Language Infrastructure and C#
Microsoft Compatible API
Run ASP.NET, ADO.NET, and Windows.Forms 2.0 applications without recompilation
Open Source, Free Software