I just saw this article on SitePoint that says:

According to recent research by W3TechsjQuery is now used on half of all websites worldwide. The popular JavaScript library has come a long way since John Resig’s Selectors in Javascript blog post in 2005.

Perhaps of more interest is the fact that, of all sites using a JavaScript library, 88.3% opt for jQuery. It’s nearest competitor, MooTools, is far behind with less than 9%.

The most interesting part of the article to me is that 50% of sites that use other libraries like MooTools also use jQuery. They speculate that it's to get a specific effect and I would guess that's the case. It would be cool to see what large sites are using jQuery as well.

http://www.sitepoint.com/jquery-used-on-50-percent-of-websites/