CSS Minitabs Pull-Down Menus
aplus.co.yu has taken mini-tabs to the next level by extending them into fly-out (drop down, hover, pull-down) menus. Unlike the previous pure CSS pull down menus mentioned earlier on the pile, these work in Win/IE (6, 5.5 & 5.0). To learn more about the effect, read the aplus thread on the topic. It gives a great, step-by-step explanation of the various elements that comprise the menu system.
Update: I recently stumbled upon {style:phreak;}'s version of a similar type of menu, unrelated to mini-tabs: CSS Only Branching Menu