I am helping an organization revamp its corporate websites. They had a scattered style of development during the late 90s and early part of the 2000s. Each department had its own site. They are now rebuilding. They have a big focus on cleaning up and consolidating. They want to commit to one server language (PHP) and one framework. As much as possible, they are trying to reduce the number of tools they are using.
Artur Kim answers:
According to recent surveys and search results, jQuery is the most popular.
YUI, MooTools, Dojo: <10% each
Yahoo! YUI: 3%