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Ruby is increasingly used for Cocoa development. Ruby has a Smalltalk-like object model with Perl-like syntax. The object model means that it is a good fit for Cocoa, although the syntax is an acquired taste.
There are two versions of Ruby available for OS X. RubyCocoa uses the standard Ruby implementation and provides a bridge to Objective-C, allowing the use of Objective-C objects. MacRuby is a new implementation that compiles Ruby objects to use the same underlying object model as Objective-C.