RubyCocoa is a nifty "bridge" between Ruby and Objective-C (the language of Cocoa), that allows developers to "write full-stack Cocoa applications in Ruby" or a "Cocoa application that mixes Ruby and Objective-C code." It has just received a major update to version 0.11.0. The release notes list the considerable changes; these include compatibility with Ruby 1.9, support for Quartz Composer, support for 64-bit applications and much more. As it happens, I've been slowly learning Ruby, so I'm looking forward to learning how to use RubyCocoa to build Mac applications with Ruby on the back-end.
RubyCocoa is free and open source; it's a free download from SourceForge.