Sean Lyndersay over on the IE team blog posted about the RSS Simple List Extensions (SLE) and how they are being used today.
(Note you need IE 7 to see the SLE functionality in-browser. Since not everyone will (or can) try out the IE7 beta, I've created some screenshots so you can see what all this is about.)
The first is Amazon's use. Here is Jeff Bezos' wishlist as an RSS feed with SLE.
Then there's eBay - this RSS+SLE feed provides filtering and sorting on a number of different categories:
Yahoo! Music has been experimenting too. Here is the Yahoo! Music Top 10 Songs list in RSS+SLE
More links including videos over at the IE7 team blog.