Doing periodic polling in web apps without leaving/reloading a page via Ajax-style programming is something I'm quite excited about.  It'll be cool to have a web page auto-update by way of a VB-esque timer or JavaScript window.setInterval() or other related service when inventory runs out, or a document isn't being edited, or a user logs into the system, or something like that - without post backs, refreshes or redirects.

Michael Mahemoff has a great blog about this very topic, talking about such features in Ajaxian chat services and multiplayer gaming.