I've had a tip off to check two killer apps.
First is
JQuery. It was plugged to me as the new mootools!
Completly supporting older browsers, it's on-site samples and documentation are ment to be excellent. It is also compatible with Google's JTools.
Second is
IETester. Useful if you need to test your sites on multiple IE versions and their equivilent JavaScript engines. It is a "free
WebBrowser that allows you to have the rendering and javascript engines of
IE8 beta 2, IE7 IE 6 and IE5.5 on Vista and XP, as well as the installed IE in the same process."