MonoDevelop (OSX, Windows, Linux)
MonoDevelop is an opensouce IDE, specifically tailored for working with Mono/.Net. It features advanced IDE features like auto completion and code insight. It supports C# and Boo.
OSX support is still experimental and doesn't currently include Boo support. There's also currently no official installer for Windows.
Language Support Table
* Additional bundle required (see above)
Operating System Table
* OSX and Windows support still preliminary