The comments also introduce jQuery which looks quite interesting.
Update: 20061126, 0230
As my colleague Sudhindra pointed out in his comment, I did miss out behaviour.js which allows you to write much cleaner HTML. Funny he mentions it, because I have a post on a behaviour.js hack to reapply styles to dynamically added or modified DOM elements - something which most AJAX apps tend to do - sitting in the Drafts section of my blog for almost a month now pending my figuring out how to post HTML markup on blogger (something I complain about in my very next post!).