What's the best way to send "events" from server to client?

Question

I was curious as to the best way to notify a web browser that it's time to refresh? For example, if you have a web application with 6 sessions open and one of the sessions makes a major change that the other 5 sessions should know about... what can you do? Do you HAVE to wait for a postback to update the interface to let the user know? Do you have to have a client-side script "poll" the server every dozen seconds or so? Any insight into this?