Royal Pingdom recently released a survey of 200 popular websites detailing which sites use which Javascript frameworks. The list of sites was drawn from the Alexa US Top 100 and Webware's Top 100 Web Apps. Using an automated tool, they checked for the presence of the following frameworks: Prototype, JQuery, MooTools, Yahoo! UI Library, Dojo, ExtJS and MochiKit.
The survey found that of the sites visited, only four frameworks were found to be in use: Prototype (with 13 sites employing it), JQuery (11 sites), Mootools (4 sites), and Yahoo User Interface Library (7 sites). Some sites used more than one of these frameworks. According to Royal Pingdom: "The ones using more than one framework were Digg (Prototype and JQuery), Bebo (MooTools and YUI) and YouSendIt (Prototype and YUI)." Royal Pingdom offered this disclaimer concerning the survey results:
It should be noted that this survey doesn't necessarily give a 100% complete picture since we only looked at the homepage of the websites. We also didn't log in to any websites. And of course, we didn't look for every single Javascript framework out there.
A detailed breakdown of the results is posted on the Royal Pingdom blog.