Here's a bunch of the best open-source JavaScript libraries on the Web. JavaScript is a key part of AJAX, and AJAX is a key second and third-generation Web technology (I won't use the buzzword you know I'm thinking of).
I recently emailed this list to a colleague who mentioned that JavaScript can be a bit awkward to deal with, especially coming from the .NET or C/C++ world. These free code libraries will indeed help, as most of them provide a standard or codified syntax for dealing with the Document Object Model (DOM), AJAX, XML, and other key technologies, especially across the browsers. Enjoy!