One for Mac users who want to do more with C# using Mono. MonoObj is a bridge that simplifies calling the various Mac OS X libraries (from Mac OS/X 10.5 and up) from .NET applications that run under Mono in any .NET language, including C# of course. The OS X libraries include WebKit and Cocoa. There's also a set of addins for MonoDevelop (the free Mono IDE) with templates for Cocoa, Console applications and others.
You need Xcode to design the UI but that's free anyway. There's a clever bit of bridging the worlds of Objective-C objects and .NET objects with a ,NET wrapper generated for every Objective-C object. Quite neat!
- Link to Code Library for Mac OS X