HTTP Long Polling, while a little bit of a hack, works incredibly well for most cases. Add in WebSockets and WebRTC in more modern browsers, and you're able to

HTTP Long Polling, while a little bit of a hack, works incredibly well for most cases. Add in WebSockets and WebRTC in more modern browsers, and you're able to do pretty much any realtime thing you can think of (if you design a server to support it).