[Javascript] How to interact with a popup created by a different page?

I have a popup window that can be opened from any page on a site. For the purposes of explaination, let's say it's a shopping cart display. Whenever someone adds an item to the cart from a page, I want to use Javascript to tell the popup to update itself.

The way this site works, there may be many pages open that can select items in different browser windows at the same time and they all need to be able to tell the popup to update itself. As a fallback, the popup will use AJAX to periodically check for updates, but if it could update instantly, it would be better.

Any ideas?