Here some of my current favories.
http://www.jquery.com (Nice small tool for programming interactivity into your pages easier)
– jQuery supports CSS 1-3 and basic XPath.
– jQuery is about 10kb in size.
– jQuery works in Internet Explorer, Firefox, Safari, and Opera.
http://jstween.blogspot.com/ (Very well written, and small, library for adding motion tweens to any positioned object in the DOM. We are using it not in a couple of places for Account Management)
http://microformats.org/wiki/rest/ahah (Useful tool when AJAX is overkill, but you want to centralize some content. It basically allows you to pull in a static HTML snippet and append it to any node on your page. Has a much smaller footprint than the AJAX equivalent.
http://moofx.mad4milk.net/ (Accordion widget that works great in every browser)
http://prototype.conio.net/ (Another framework)
Good times for an interaction designer.