Mac OS X: Assuming you're not doing anything with the very corners of your screen, free (for now) utility MaxMenus has a use for them—customised menus of recent documents or applications, mounted volumes, system preferences or other shortcuts.
MaxMenus comes from the same makers of recently featured LiteSwitch X, and is available under the same conditions—it's free right now, while in beta, but may become paid software at some point. It's fairly simple to set up and start using. First, head to your System Preferences and tell MaxMenus where you want to activate your menu. The corners are the convenient default, but you can place a menu elsewhere, and have it spring-load all kinds of useful stuff:
When you mouse close to a screen corner, a barely-noticeable green blip appears at the very edge. Click it, and down drops your applications or whatever you wanted to keep convenient.
MaxMenus is a free (for now) download for Mac systems only.