Yes, there is Mootools, Jquery, prototype and YUI among others.. I have used them all with xajax succesfully. Lately I've been using YUI and xajax together. I use xajax for the server (and client side also) and YUI modules for the CSS,Effects, Layout, Calendar, Drag & Drop etc. modules.
Those all libraries have also ajax functions but xajax does it so well(you just create normal PHP functions and call them from the script like any other js function). Normally if you use those other libraries, ajax is done so that you create a php file(s) and you code the functionality based on GET or POST variables.