Cocoa bindings allow .NET/Mono programs to call native Mac OS X APIs. This is usually needed to create a native-looking (and native-being) GUI.
Just a few days ago Novell announced MonoMac, the newest addition to the team:
I myself use Monobjc. The first library to do this job was CocoaSharp.
These are the Cocoa binding libraries for Mono that I have heard of:
CocoaSharp (deprecated, part of Mono)
Cocoa# (CocoaSharp) is an effort to bring Mono and the Mac OS X Cocoa toolkits together in a two way bridge, allowing Mac developers that leverage the Mono runtime libraries, while retaining a native Mac look and feel. This bridge also allows Mono based applications to have access to the Cocoa libraries directly.
CocoaSharp is still part of Mono but hasn't been in active development for years.
Monobjc (active, not part of Mono)
The Monobjc bridge provides the necessary tools to develop and run .NET applications (written in C#, VB.NET, etc.) that interact with Objective-C frameworks and libraries under Mac OS X. Existing Objective-C classes can be used in .NET code in an almost transparent manner.
Since Monobjc is not part of Mono any program that uses Monobjc will have to be distributed with the Monobjc dlls.
mobjc (active, not part of Mono)
mobjc is a Mono library which allows .NET code to interoperate with Apple frameworks such as foundation and appkit. It makes it possible for managed code to call native Objective-C methods, native methods to call managed code, and new Objective-C classes to be defined in managed code. Exceptions are properly marshaled in both directions.
mcocoa (active, based on mobjc, not part of Mono)
mcocoa provides a set of wrapper classes which allow Apple's Cocoa frameworks to be used from Mono. The wrappers are generated using the make file and rely on the mobjc library to handle the bridging between Objective-C and Mono.
MonoTouch (for iPhone OS)
MonoTouch allows developers to create C# and .NET based applications that run on Apple's iPhone and Apple's iPod Touch devices, while taking advantage of the iPhone APIs and reusing both code and libraries that have been built for .NET, as well as existing skills.http://monotouch.net/
MonoMac (announced for Mono 2.8)
I could find a catchphrase paragraph for MonoMac. But a blog post explaining what MonoMac is can be found here: