We've just open sourced a simple chat server we wrote a few months ago, Cockatoo.
It's using the fantastic EventMachine (yep, we know that node.js is the new kid in town, but well...). Several "push" technique exists, but we focused on Long Polling as it's very easy to setup, be it on the server or the client side. Screenshots :

We've also made a little presentation on "Long Polling with EventMachine" : [slideshare id=4354968&doc=longpollingeventmachine-100530102731-phpapp01]