Google now offers a cached compressed copies of Prototype, script.aculo.us, jQuery, Mootools and Dojo on its high-speed content distribution network via the AJAX Libraries API. Inclusion can be done directly (viz. <script src="http://ajax.googleapis.com/ajax/libs/prototype/1.6.0.2/prototype.js"></script> or via Google AJAX API Loader’s google.load() method. Neatest part of google.load() is the smart versioning feature. Very nice!