There are a few, but none of them are widely known about. Socket.io's problems only become visible once you've been working with it for awhile.

The three alternatives I know about are SockJS: http://sockjs.org

Faye - which implements the Bayeux pubsub protocol: http://faye.jcoglan.com/

And node-browserchannel (mine) which implements google's browserchannel protocol: https://github.com/josephg/node-browserchannel (BrowserChannel works all the way down to IE5.5!)