While gathering more information on AJAX (see ajax.marcocantu.com for some of my demos) I've come across a huge number of alternative JavaScript libraries. With interest in the language soaring after the Google and others are pushing nicer and nicer web-based applications, the offering in libraries (mostly open source) is very high and picking a good one is hard (or more than one, as they differ in the domains they are best applied to).
For a good overview of the more popular libraries, I can suggest you reading the JavaScript Libraries Worlcup (clearly influenced in the title by the Football World Cup that was taking place when it was written) by Dan Webb. The article compared 4 popular options:
Another very interesting "comparative" article, even if one not so detailed, is on Simon Willison blog. After browing the web a lot more, I ended up picking a candidate for my future AJAX-related efforts, jQuery. It is small, simple to use, very CSS-oriented, and practical. There are extra pug-in controls as well. Like many others, it is oriented to graphic web pages more than to data-entry pages (which is what I need most), but it looks very interesting. I'll let you know how I progress using it.