Mouseover Pop-up?

I am trying to figure out a way to display page content, or a widget in a lightbox-ish sort of way. It's difficult for me to explain.

Check out http://www.fastenal.com add something to the cart, and then see how you can mouse over to see the contents?

Or Check out http://www.digg.com and look at the tab on the left for the feedback.

I want to possibly display my shopping cart contents in that fashion but cannot for the life of me figure out how. I've tried googling but apparently the keywords I'm using do not suffice.

Any help would be awesome!