UPDATE: StackExchange.Netgain is now open source!!
We use a custom socket server that we developed in house. It supports every historic + RFC version of the web-sockets api; not just Hixie 76; hixie-76 === hybi-00; hybi-4/5/6/7/8, and hybi-13 === RFC6455. We treat sockets as a value add for those who use modern browsers and don't rely on it to provide all updates to our sites as websockets aren't supported in all browsers at this point.
We developed our own very simple protocol on top of websockets to provide rudimentary updates to question lists and as well as other interactions. We use redis messaging to support a unified messaging bus between all of our socket servers. The server side is a Windows service written in C# and the client side is jQuery.
There was talk of open sourcing our implementation but haven't reached a release yet. I have been very impressed with the load we've been able to handle thus far.
Here's a snapshot of haproxy at the moment:
In case you're wondering - that is 128k simultaneously connected web socket clients all receiving updates via our unified messaging bus.
Note by Marc Gravell♦: if people would be genuinely interested in our releasing this as open source, please let us know. It will take some effort. but isn't impossible.