Rotate pictures 90 degrees counter-clockwise in iPhoto, Photos, and Preview. If you want to rotate the other direction, hold the Option key.
The Mac's Contacts app reveals the Groups a contact is in, at the press of a key. Of course it's the Option key. This will help you one day.
Close all of your open Finder windows with a keystroke. You know about Command-W; try adding the Option key. Or, Option-click a Close button.
Hold the Option key while clicking on the Battery Status menu on your Mac laptop and find out whether your battery is in good shape or not.
Hold the Option key, then click on the WiFi menu to see some interesting stuff about your network. Handy for trouble-shooting. Try it!
Your user "Library" folder is really important-- so important that Apple hides it. Here's how you can find it and open it up.
Cut your work in half in the Mac OS Messages app. Deleting conversations used to be click-the-x, wait for a box, then click Delete. Now it's one click.
Apple's System Information app can be accessed by going to the Apple menu while holding down the Option key. You save one click this way. And you look cool.
Skip the "Are you sure?" dialog box when emptying the Trash by holding the Option key down while you do it. Also helps when something won't delete. Try it!