Pushcar is a project mainly written in Ruby, based on the MIT license.
Comet (ajax push) inside of rails, based on pusher, using thin + eventmachine.
This is a derivative work. All of the hard stuff was done by macournoyer for pusher ( http://github.com/macournoyer/pusher ).
I've been hacking it up to make it entirely within Rails, instead of as middleware, and making the interface more Rails-ey. The advantage of this is that you can mix comet endpoints into your app and place them behind authentication, etc.
Right now, the message queue is in-memory and blocking. Stupid and unscalable, but it works for now. The strategy was "get something working, settle the API, then improve internals." Hang in there.