Hold the Option key while tapping volume up, volume down, or Mute on the keyboard to quickly access the Sound preferences.
Sometimes the Finder's Favorites disappear (this happens in iTunes also. Here's how to bring them back. Does not require programming.
Apple has moved the Software Update options again! Here's how to find them. It's important to know where they are so you can change them as desired.
Name your Mac, making it easier to find it when using file sharing or AirDrop. This is especially nice if your Mac used to belong to someone else. Make it yours by renaming it.
Bring some color back to your Mac! If the Close, Minimize, and Zoom buttons aren't red, yellow, and green, check the System Preferences.
Minimize a window with a simple double-click in the title bar. It's easier to click the title bar twice than to click the yellow button once, because the target is so much bigger.
Quickly find the proper item in System Preferences, by searching. There's a reason they put that "Search" box up there-- try using it! Great for new Mac users.
You can preview your Screen Saver choices right there in the System Preferences app. It's much quicker to do it this way.
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.