One common pattern I've seen in usability, is that users don't always have great control over their mouse. It's easy to accidentally overshoot a target area with you mouse, which is really frustrating with things like nested dropdown menus, where accidentally mousing away from a target area may end up collapsing/closing the open menu.
Giva's recently released the MouseIntent jQuery Plug-in which aims to give developers a way to control this behavior. It works by monitoring an invisible border around the element to see what the user appears to be doing. If a user quickly moves back into the original element, then then no mouseaway event is ever fired. The plugin has a number of settings that allow you to control the behavior—such as monitoring whether or not the user is still moving the mouse in the invisible border area.
I've used the plugin in one of our dropdown menus that has nested menus and it's really helped to improve our user's experience.