JQuery and more...

JQuery is great to write RIAs (Rich Interface Applications). Its light and one can do everything from handling easily HTML to AJAX.
Add to that some great plugins out there and one can easily write RIAs.
Here a couple of good links for plugins:

http://www.smashingmagazine.com/2009/01/15/45-new-jquery-techniques-for-a-good-user-experience/

http://www.noupe.com/ajax/50-amazing-jquery-examples-part1.html

Also, if you are planning to build a CMS using AJAX this is a great library to use
http://extjs.com/
Its a very powerful library here are some samples of its capabilities:
http://extjs.com/deploy/dev/examples/samples.html

Recently it was announced that Microsoft and Nokia will be adopting JQuery. In particular, JQuery will be bundled within Visual Studio.