I've been going crazy with this XMLHTTPRequest stuff recently. One of the coolest things you can do with XMLHTTPRequest is juice up your old search forms with LiveSearch.
I installed a LiveSearch module for drupal the other say (see search block to the left), and then decided it would be neat to implement livesearch on my FeedsOnFeeds powered news aggregator. With lots of help from the LiveSearch wiki and some custom hackery on my part, the image on the right is what I came up with. Complete with favicon eye candy, I now can search my server side php news aggregator. Cool beans.
I'm doing a presentation on XMLHTTPRequest sometime in the next couple of weeks. You can view the slideshow I'm working on for the talk here. It's not so much right now, but I'll post a link to the whole thing and details about the presentation when it's all done. The slideshow uses s5, Eric Meyer's wonderful Simple Standards-Based Slide Show System.