JQuery
JQuery is a Javascript framework for interacting with the DOM, doing AJAX requests and lots of new modern things.
Checking JQuery version
$.fn.query
will return the current runtime JQuery version.
Loading multiple JQuery versions in a page
From http://forum.jquery.com/topic/multiple-versions-of-jquery-on-the-same-page, you can use jQuery.noConflict() to load multiple versions:
<script src='jquery-1.3.2.js'></script>
<script>
var jq132 = jQuery.noConflict();
</script>
<script src='jquery-1.4.2.js'></script>
<script>
var jq142 = jQuery.noConflict();
</script>