Google Code begins hosting AJAX Libraries
Google are using their CDN to host a number of Javascript libraries (including jQuery, prototype, and others). This means that a browser should only have to download the code once, rather than once for each site they visit that uses it.
You can link directly to the .js files, or use a Google API call to always load the latest version of a library.
http://code.google.com/apis/ajaxlibs/