This page shows instructions on how to start using PlainAjax on a website and directs you to some working examples.
All thats needed is to include plainajax.js source file in the HTML header:
<script type="text/javascript" src="plainajax.js"></script>
plainajax.request('respurl: serverscript.jsp; resultloc: myhtmlelement;')
Check an example on how to