Message Queue for Rails?

Use your favourite jabber/XMPP server and you immediately have a messaging
system with authenticated entities, status updates, queueing of messages
when a server is unavailable, etc. And even better, you can add your
servers as contacts in your favourite chat client and send them instructions

Here at Twitter we use an in house, open source solution, called
Kestrel which has great performance.

Robey Pointer wrote it, and it's quite good. It uses a simple memcache

Set = enqueue. get = dequeue.