Hello
Prototype is a popular JavaScript library that provides many utility methods that simpify common tasks, including using AJAX. You can obtain Prototype.js and learning materials from prototypejs.org.
Scriptaculous is a popular JavaScript library built on top of the Prototype library, that adds tons of special visual effects. You can download Scriptaculous.zip (which includes Prototype) and obtain learning materials from script.aculo.us.
Other JavaScript libraries include Dojo and Rico (based on Prototype).