You are asking about an important feature which is unfortunately not so easy to implement with HTTP, which is primarily build for pure client-server paradigm, when everything happens on request from client side: a client sends HTTP request, and get HTTP response from server.
This is called
pull technology. It is really a draconian limitation. Even such a basic thing as a Web chat suffers greatly from such limitation. A client has to constantly poll the servers for new events on the char, which is wasteful and not responsive enough from at the same time. What you really need is so called
server push. Please see:
http://en.wikipedia.org/wiki/Pull_technology[
^],
http://en.wikipedia.org/wiki/Push_technology[
^],
http://www.asp.net/signalr[
^],
http://signalr.net/[
^].
SignalR referenced above is a new ASP.NET library implementing server push.
—SA