Jello

Because everybody likes "paste & jello" sandwiches, right? I know I did when I was a kid.

Jello is a simple library to watch the OS X pasteboard and do something on every paste.

require 'jello'

Jello.new do |paste|
  system "say 'You pasted #{paste}'"
end

For example, to watch for URLs copied, and then shorten the URL and replace the long URL with the shortened one:

require 'open-uri'
require 'jello'

Jello.new :verbose => true do |paste, board|

  case paste

  when %r%^http://.*%
    uri = $&
    uri.gsub! /#/, '%23'
    next if uri =~ %r%^http://bit.ly%

    shortener = 'http://bit.ly/api?url=' + uri

    short = open(shortener).gets.chomp
    board.puts short

  end

end.start

A few things to note:

In the block context, #puts and #gets are commandeered for the pasteboard object — use STDOUT.puts and STDIN.gets respectively if you need terminal interaction in the block context to be safe.
Creating a Jello block doesn't actually do anything by itself - you need to act on the paste itself. Test against contents and conditionally run code, whatever you want.