Socket.io is a great library. When it comes to a contender, I see Server-side Events [1] as a contender to WebSockets. I feel it's a simpler architecture which fits HTTP/REST better than WebSockets in the general case. I built a couple of small experiments/examples using CoffeeScript and Redis [2]; the middleware.coffee in particular is designed to be used together with REST, where if you send "Accept: text/event-stream" you will get updates to the resource(s) on that URL.
On the plus side, it's supported by Opera Mini and iOS.
For IE and others (Android, I'm looking at you!), I have yet to try it and so can't vouch for it, but there exists a polyfill: https://github.com/Yaffle/EventSource
1: http://dev.w3.org/html5/eventsource/ 2: https://gist.github.com/1560654