Infoworld reviews 6 of the 'top' open source AJAX libraries. What I picked up from this article is that although we are still in the early stages of AJAX, there is enormous potential, and some great toolkits that will help you get started using AJAX on your webpages. For my first foray into AJAX, I didn't use any of the abovementioned libraries. I used scriptaculous to create a drag and drop sortable list that is automatically updated in the database. This is such useful functionality - so much better than having to re-order items by moving them up or down one row at a time. I have recently used the Yahoo User Interface Library (YUI) to create a tree view of publications for a client. What I like about the YUI is that it has good documentation and examples, plus it provides you with some base style sheets to allow you to standardise your design and allow for consistency across the different browsers.
Check out the toolkits: