SharpDevelop it’s an Open Source IDE for C#, VB.NET, Boo, F#, IronPython and IronRuby on Microsoft’s .NET platform.
With the release of version 3.2, the development team included full support of IronRuby 1.0 which is also an Open Source implementation of the Ruby programming language targeting Microsoft .NET Framework, heavily relying on Microsoft’s Dynamic Language Runtime (DLR).
SharpDevelop 3.2 currently supports .NET Framework 3.5, the support of .NET 4.0 will be part of SharpDevelop 4.0 which will be released soon.
So, what are you waiting for? Get your hands on SharpDevelop.