CornerClick is a nice freeware program that lets you assign actions (including AppleScript) to the screen corners (with or without modifier keys) in Mac OS X Panther and up.

It worked well for me on Tiger, except that I couldn’t assign the Exposé and Dashboard actions, but I imagine that’s because it conflicts with Tiger’s built-in System Preferences for mapping those to corners? I did manage to map “Hide Other Applications” though, which I tend to forget the shortcut key for (it’s Alt+Command+H, in case you’re curious).