The
option key is the
Mac's principal
command modifier key. Not only does it turn
e into
é etc., it invokes alternate modes on a host of other
system and
application commands.
For example, command-W closes the frontmost window. Command-option-W closes ALL windows in the frontmost application. Command-click on a window's snap box to snap the window up to only its title bar; command-option-click to snap ALL windows to their title bars.
Many uses of the option key are not documented. That's the Macintosh Way: understand how something works in one program and there's an excellent chance it will work similarly in others without your having to RTFM. Once you realize how ubiquitous the option key is you check for it instinctively and find all sorts of Easter eggs. Credits in Photoshop's About window scrolling by too slowly? Press the option key and they go by twice as fast. Command-I gets info on any selected object, but what if the item is an alias? Command-option-I gets info on the original!
My most-used option trick, though, is this: option-clicking on the desktop hides the frontmost app without a trip to the Application menu. For people who prefer to use the keyboard over the mouse whenever possible, Apple's implementation of the option key is just plain sweet.