Prototype.js - A Javascript Framework

Prototype.js is a "javascript framework". It is a very popular set of Javascript functions and objects that add extra functionality to the basic Javascript commands available in most browsers. Prototype is useful on at least three levels:

1. it simplifies many things that are normally very complicated in Javascript
2. it provides a single cross-browser compatible way of doing many routine task that used to have to be done differently for different browsers
3. Javascript is erratic, moody, and idiosyncratic. Prototype provides a consistent, "reasonable" set of interfaces that act as you would expect them to.
   

Here are some useful links:
The official prototype.js site
An unofficial (but very popular) guide to prototype.js