I've mentioned AjaxRain (http://www.ajaxrain.com/) on numerous occasions and for good reasons. They have by far, the most comprehensive collection of Ajax/DOM/JS components around and its constantly being updated. Apart from that, they're a jQuery-powered site with a developer that loves jQuery and is always posting the latest jQuery plugins on there.
They're now up to 418 plugins and demos so be sure to check them out: http://www.ajaxrain.com/ Rey
