RubyCocoa is a bridge between the Mac OS X Cocoa Framework API that makes it callable from Ruby code. You can create applications, objects, call Mac OS X Services. You can even call Applescript:

require 'osx/cocoa'
include OSX
def speak (str)
  str.gsub! (/"/, '\"')
  src = %(say "#{str}")
  NSAppleScript.alloc.initWithSource(src).executeAndReturnError(nil)
end
speak "Hello World!"
speak "Kon nich Wah. Ogan key desu ka?"

Ben Bleything submitted a talk to RubyConf, titled “Harmonize: Exploiting RubyCocoa and Sync Services for Fun and Profit” where he discusses making a wrapper around the SyncAPI in Ruby. (blog post introducing Harmonize)

RubyCocoa looks like it may have stagnated a little as the last release was November 2005, but perhaps with such a renewed interest in Ruby, and Mac OS X gaining popularity, perhaps new life with be breathed into it. Projects such as SyncBridge and Harmonize also obviously help.

Update: Check out the very good examples and information at RubyCocoa.com, including how to use your Apple Remote from RubyCocoa .