It does what the title says. If you click on the header of a category, it will collapse, and if you click it again, it will expand. I don't know how many browsers it will work in, I know it works in FF though. It's just something I whipped up very, very quickly.
<script>
/*Toggle Category Headers By Vitality of TCZ || Revolution X*/
$(".catbg").click(function (){$(this).next().toggle();});
</script>