Some very useful keystrokes you might not know about.

This is the feature of the Mac interface I hate the most: secret key combinations that invoke useful functions.

Here is an Apple support document listing secret key strokes: OS X:keyboard shortcuts.

To put the display to sleep: press the power button for a second, or in Sierra and later, ⌘ ⌃ power (because newer Macs do not have the eject button). Before Mavericks, use ⌘ ⌥ ⏏.

To kill a hung application: ⌘ ⌥ ESC.

How to force a shutdown if the computer won't shut down normally: hold power key 10 seconds.

Booting the Computer

When booting the computer:

• Hold Shift to start up in Safe mode.
• Hold Option ⌥ to enter the Startup Manager, which lets you pick the startup disk.
• Hold D to start Apple Hardware Test.
• Hold D to start in Target Disk mode.
• Hold ⌘R to boot from the Recovery Partition.
• Hold ⌘⌥PR to reset the NVRAM or PRAM (Intel Macs).
• Hold ⌘S to start in Single User mode.
• Hold ⌘V to start in Verbose mode.

Apple Menu

Choose  ► Restart while holding Option to avoid being asked if you REALLY want to restart.

After restart, an app may be unable to re-open its previous windows, and be unable to launch. Hold Shift while launching the app; this clears out its "saved state". (Thanks, MacInTouch and Matt Neuburg.)

Finder

Control-click brings up a context menu: this is the same as right-click on a two-button mouse.

Quick look: in finder, select a file or group of files and hit the space bar, and you'll get a popup window with a preview. Works for Mail.app attachments also.

Desktop

Hit ⌘space for a Spotlight search.

Screen shot. ⌘ shift 4 gets cross hairs, hold mouse, Drag, release. Picture is stored on Desktop.

Hit F9 to see thumbnails of all windows, F11 to hide all applications and see the desktop.

Dock

Right-click on the dividing line in the dock: this brings up a menu that lets you change Dock preferences such as hiding and magnification.

Applications

Control-click (or right click) on a word in some apps and choose "look up in Dictionary."

iTunes

⌘-shift-M toggles the mini-player in iTunes 12