The boo language binding for the free SharpDevelop IDE was created by Daniel Grunwald.
SharpDevelop for .NET 4 is a stable and excellent open source IDE for the Windows platform. It includes the boo addin, which has more new features like refactoring, renaming, and a forms designer. (If you would like to use SharpDevelop 2.2 with boo 0.78 and later, please follow the following instructions:
Download the SharpDevelop source code, put the new boo version into SharpDevelop\src\AddIns\BackendBindings\Boo\RequiredLibraries, then run SharpDevelop\src\AddIns\BackendBindings\Boo\RequiredLibraries
Note: Compiling SharpDevelop does not require any tools except the .NET Framework 2.0.
The code converter (converting C# to boo) is much improved, and it is also accessible online via a web form.