A better Category menu

My site is located a http://www.thommcneilly.com. I am quite happy with my first wordpress site so far, but the more content I add, the more categorys that are going to be displayed. The problem this causes is eviden, so I was wandering with anyone knoew a good plugin I can use to show the main categorys, and then you cansee the sub-categorys when you hover over it?

My cetegory section is called books by the way 😀