Wine.com has recently leveraged the power of ASP.NET AJAX to improve the user experience for customers visiting their site.  One example is that by updating the shopping cart without a full page refresh, the process of adding new items to the cart is made seamless.  By implementing this simple solution, the site has improved the overall User eXperience for buyers making online purchases. 

Screenshot - Wine.com

A less obvious, subtle change is the description that appears when clicking on the icons associated with each wine (e.g. Professional Ratings).  Rather than using an 'obtrusive' new browser window (which popup blockers may intially deny), definitions appear in a popup window directly on the page.  If a user were forced to use a new browser window or tab, the focus on the original page could be lost resulting in a less than ideal UX.

ASP.NET AJAX makes it is easy to add these kind of improvments to your site.  It would be interesting to analyze how many e-commerce sites incorporate AJAX functionality or are planning to use it in the near future.  Out of curiosity, has anyone seen a site implement an address form that does an auto lookup on a zip code and automatically populate the city and state? (I don't recall who mentioned this idea, but I think it would be nice subtle feature:)