ASP.NET AJAX has some pretty nifty features, but one of the nicest is the UpdatePanel, which allows coders to easily set up partial-page refreshes through XMLHttpRequest, and from what I can see it has good cross-browser support.

However, one thing I always wanted to do was chain UpdatePanels together so that (for example) a number of server-side requests could be fired off, with the user status being updated appropriately.