Stephen Stchur has written a script to mimic IE’s proprietary mouseenter and mouseleave Javascript events for non-IE browsers. This is useful when you have one element (B) laying on top of another element (A), and you want to do a mouseover/mouseout on A without the mouseout firing when you hover on B. Quirksmode has articles on JavaScript mouse events and event bubbling that give good background information.

Update: MooTools has mouseenter/mouseleave built-in.