From Greg Schueler:
CornerClick allows you to assign actions to each corner of your screen. Your screen corners can act like four customizable buttons which will never move.
You can hide the current application or all other applications, and open any file or application. You can open an URL or run an AppleScript. You can perform Expose actions, and activate Dashboard. You can perform multiple actions with one click, or zero clicks. Each screen corner can have many different actions based on modifier keys.
A popup window can show you what action will be performed when you point at the screen corner.
You will never miss when you try to click the corner of the screen, so CornerClick lets you do things without even looking.
Supports multiple monitors.