With MacRuby, you can easily extend or wrap any Cocoa class. The class below, FormHelper, gives a more ruby like interface to the NSForm, a fairly useful control in AppKit. All of a sudden
form.cellAtIndex(1).objectValue
becomes
form_helper["Email Address"]
So much boilerplate and standard code you'd write in Cocoa can be abstracted to neat helper classes with ruby's classy meta-programming.