Here are some easy to install and essential jQuery methods:

Stop double clicks on Form Submit:

<script type="text/javascript" src="../scripts/jquery-1.4.js"></script>
<script type="text/javascript">
$("form").submit(function() {
$("submit",this).attr("disabled", "disabled");
});
</script>


Select just one Element from the list to display.

This one is displayed at phpwebdesigns.com/WebDesign/samples
$(document).ready(function() {
$("div.body").hide();
$("div.body").eq(2).show();
});

Display just the first Element in the list

$(document).ready(function() {
$("div.body").slice(1).hide();
});


Make all external links open in new window.

This one is used in this page and needs the document ready part.
$(document).ready(function(){
$("a[href^='http://']").attr("target","_blank");
});