Prototype

Prototype is one of the earlier Javascript frameworks and is also included in the Ruby on Rails framework. Of the websites in this test, a total of 13 used the Prototype framework.

JQuery

JQuery is a framework that has received a lot of attention due to its speed, size and smart modular approach which has led to a big library of plugins. Of the websites in this test, 11 used the JQuery framework.

MooTools

Just like other Javascript frameworks, MooTools contains several functions to help development. One of the more known ones is its advanced effects component. Of the websites in this test, four used the MooTools frameworks.

Yahoo! UI Library (YUI)

Yahoo has developed its own Javascript framework. They use it for their own websites, but have also made it freely available to others. Of the websites in this test, seven used the Yahoo! UI Library.

Source: http://royal.pingdom.com/?p=305