If your Mac's screen is white text on black, instead of black text on white, you may be in "Inverted" screen mode. Here's how you toggle that on and off.
Toggle between normal view and inverted view
Add the Shift key to go the other way
In a spreadsheet, tabbing goes forward... but Shift-tabbing goes backward! Also works in dialog boxes and web forms.
Essential Mac keyboard shortcuts: Command-Z, -X, -C, and -V
Mac users, if you know only four keyboard shortcuts, let them be these four! Command-Z, Command-X, Command-C, and Command-V are what make the Mac the Mac.
Essential Mac Keyboard Shortcuts: Command-A for “Select All”
The 60 seconds it takes to learn this shortcut will be paid back thousands of times. You have to know Command-A for Select All. Practice!
Stuff the cat did: toggled VoiceOver so my Mac started talking
Your Mac can talk to you, telling you which button you're about to click, which menus are showing, and so on. Very handy if you can't see well, but very surprising to have it turned on by itself. Or by the cat.
Essential Mac Keyboard Shortcuts: Use Spacebar to pause/resume audio and video in iTunes and YouTube
Use the space bar to quickly pause audio (or video) in a variety of Mac apps. Tap it again to start it again. Very handy.
Hold Command key to see file path in Mac’s Spotlight
Spotlight is great, it finds stuff for you. But when you want to know where the things it found are, it falls short. Unless you use the Command key!
Use Escape key to click the “Cancel” button
Quit mousing around! Tap the "Esc" key (Escape) whenever you want to click a "Cancel" button. Quick and easy, never fails.
Make stuff bigger with ⌘+ on a Mac
Here's how you quickly make things bigger (and smaller) in many Mac programs. Try it and see if it works in the Mac apps you use!
Mac keyboard shortcut symbols, explained
Here's what those mysterious symbols mean in the Mac's menus. When I first got a Mac I didn't know. Maybe you still don't. But now you do.