The libraries that can be used are Dojo (http://dojotoolkit.org), DWR (http://getahead.ltd.uk/dwr), Prototype (http://prototype.conio.net), and the AjaxTags component of the Java Web Parts project (http://javawebparts.sourceforge.net). All of these are free, open source toolkits that people are finding very useful in their Ajax work. This is only a very small subset of the libraries and toolkits available, but they probably represent the most popular.
Re: AJAX libraries
The libraries that can be used are Dojo (http://dojotoolkit.org), DWR (http://getahead.ltd.uk/dwr), Prototype (http://prototype.conio.net), and the AjaxTags component of the Java Web Parts project (http://javawebparts.sourceforge.net). All of these are free, open source toolkits that people are finding very useful in their Ajax work. This is only a very small subset of the libraries and toolkits available, but they probably represent the most popular.