jQuery -- Love it
If the concise API doesn't do what you want easily, or you're solving a more sophisticated UI problem that should have already been solved (tabs, dialogs, etc), then you'll appreciate that jQuery is even more perl-y (CPAN-y?) in that there is a thriving community of plug-ins you can cherry pick that solve common UI issues.
The recently released jquery UI project offers you a quality assured set of UI extensions that follow certain code guidelines, for those looking for the don't-make-me-cherry-pick, batteries-included approach offered by dojo/scriptaculous/yui.
There have been two occasions recently where I had to implement small tweaks to different web based enterprise application where it wasn't clear to me how to do it with using the vendor's supported API's, or if it was even possible. In both cases, I implemented the customizations by including the jQuery library in a global template, adding an onDOMReady event handler, and writing in some new DOM elements using jQuery in ~ 5 lines of code. Problem solved - fast. Greasemonkey-type short-cuts like this risk not being upgrade friendly, but when implementing something not supported by vendor API's you're usually off the seamless upgrade path anyway. In addition, doing it the vendor way at the Java/C#/php level would almost definitely result in more complex code, and much more of it, that would be harder to maintain/merge anyway. And finally, if the need arises, it's much much simpler to turn off a jQuery page hack by commenting out a single line unobtrusive event-handler-add than it is to undo a compiled or scripting language library modification. It's downgrade friendly.
I now reach for jQuery every chance I get. Take a look.