http://beaconpush.com/ - works in any browser; gracefully handles browsers that don't do websockets

http://pusherapp.com/ - fully functional only in browsers that do websockets & falls back to flash for browsers that don't; has PHP library on github

http://www.pubnub.com/

http://www.frozenmountain.com/websync/

http://hookbox.org/ - more of a full-server approach than an API