Google released a new search tool named codesearch specially for coders. They support also Zip files that contain code and you can find the sources for the Ajax engine by using the link:
When you open on of the *.js files you also have a navigation through the zip file. on the top there is the path to the current opened folder and on the left side you can see al the files within that folder.
Nice feature !
Google caches the zip file so be sure to get the actual zip file from http://www.mathertel.de/AJAXEngine.