According to the JQuery site (see bottom of its home page), users of JQuery include Google, Dell, Bank of America, Digg.com, Technorati.com, mozilla.org and many others.
Excerpt from the site:
I just saw this - looks useful for JQuery developers:
Visual JQuery Magazine Issue 1
Apart from the technical articles, it also has an interview with John Resig, original creator of JQuery.
The magazine issue is freely downloadable; look for the words "download it here" at the above link, or directly get it here.