Active Screen Corners
Both Dashboard and Exposé can be configured so that moving the mouse into a "hot corner" of the screen will trigger them. (It can also work with modifier keys, which will activate the corresponding function only when the mouse approaches the "hot corner" and the corresponding modifier key(s).
Options include: All Windows, Application Windows and Desktop for Exposé, and Dashboard to activate Dashboard. Additionally, Start Screen Saver and Disable Screen Saver are also available.
As with the hot corner settings, modifier keys can also be used.