First up, we'll just cut to the chase. Aside from the many-interface enhancing, time-saving benefits of Ajax, most of the time, you just simply want to "fascinate" your site visitors. It's easy to give your site an Ajaxy feel, regardless of asynchronously updating it with server-side XML, just by sprucing up your interface with some snappy JavaScripts. The easiest way to get many of these effects is to reference a JavaScript library (sometimes called a toolkit or framework, depending on how robust the provider feels the code is). A few of the leading favorites in the AJAX community are:
jQuery: http://jquery.com/
Prototype: http://www.prototypejs.org/
MooTools: http://mootools.net/
There's also the following:
Script.acilo.us: http://script.aculo.us/
Moo.fx: http://moofx.mad4milk.net/
And then there's:
Dojo: http://dojotoolkit.org
YUI (Yahoo User Interface Library): http://developer.yahoo.com/yui/
Prototype and MooTools are both pretty solid little frameworks (here "little" is...