Great Uses of AJAX

There is a lot of talk about AJAX these days, but very few sites are doing a truly great job of implementing this technology in a seamless and "natural" manner. I'm keeping a list here of sites that do a good job of presenting their sites with more than just "oh/ah" factor and actually make this technology integral to a great user experience.

1. http://www.pandora.com/ Uses AJAX extensively and, some might say, gratuitously, but to nice effect.
2. http://www.slashdot.org/ The usually gee-wiz-phobic site's Threshold Slider is pretty nifty. Dead simple, quick and it improves your experience and allows instant feedback on your interaction with their site. Nice.