Twitter releases the code for the messaging system they use across their clusters(rubyforge.org)

  Starling - a light weight server for reliable distributed message passing.

  = Synopsis

  # Start the Starling server as a daemonized process:
  starling -h 192.168.1.1 -d

  # Put messages onto a queue:
  require 'memcache'
  starling = MemCache.new('192.168.1.1:22122')
  starling.set('my_queue', 12345)

  # Get messages from the queue:
  require 'memcache'
  starling = MemCache.new('192.168.1.1:22122')
  loop { puts starling.get('my_queue') }