The shift key can be incredible helpful when added to keyboard combinations that you may already know. Think of it as a modifier key.
- You can scroll down a page in Safari by hitting the SPACE key. If you press SHIFT + SPACE you'll scroll back upwards.
- To see things in slow motion (this is purely just because it's cool) hold down SHIFT while you do things like open a stack from the dock or minimize a window.
- COMMAND + TAB invokes the App Switcher and cycles through the apps from left to right. Adding SHIFT to that combination will change the direction to right to left.
Those are just some examples. See what else you can come up with and leave them in the comments below...