http://www.html5rocks.com/en/tutorials/eventsource/basics/
SSE is realtime server->client protocol that's fully compatible with HTTP and has lower handshake overhead (no Upgrade roundtrip, doesn't require HTTPS to go through proxies).
It's also not supported by IE yet: http://caniuse.com/eventsource
Compare with http://caniuse.com/websockets
Looks like Chrome and Firefox have had support for quite some time. I'm alright with that.
Probably long-poll, a single connection from client to server kept alive with the occasional byte sent one way which sends the message and disconnects when there is something to send (the disconnect, requiring a new request from the client is because some clients won't process what has been sent immediately unless the connection is closed).
I once wrote a messaging client that operated this way. It is efficient in terms of bandwidth use and client footprint, though if you are using a process based we server it can be quite inefficient there (an event driven service on few processes/threads is far more efficient for handling many long-lived low-activity connections).