1. Press C during Startup to boot to a macOS X or other bootable disc.
2. Press D during Startup to use Apple's Hardware Test utility from a boot disc or built-in your Mac (Intel-based Mac).
3. Press Option (⌥) + Command (⌘) + P + R to reset PRAM (parameter RAM) and NVRAM (non-volatile RAM), two areas of memory not cleared when the computer is off.
4. Press Option (⌥) during Startup to show a boot screen with all bootable devices listed.
5. Press N during Startup to attempt a network boot or ⌥ + N to start from a NetBoot server.
6. Press T during Startup to retrieve files using Target Disk Mode when the computer refuses to boot properly. For more information see the Apple support information.
7. Press Shift (⇧) during Startup to boot the Mac on safe mode. It will disable all non system fonts, all startup items and login items.
8. Press Command (⌘) + V during Startup to boot in Verbose Mode. It will show a terminal-like interface to give information important to startup.
9. Press Command (⌘) + S during Startup to boot the Mac in Single User Mode. This mode allows you to login and interact with the computer via text input only (no graphical interface).
10. Press Command (⌘) + R during Startup to access the Mac OS X Utilities.
A last useful utility is to Press Eject, F12 or Hold Down Mouse/Trackpad Button when restarting. This allows you to eject discs stuck in the optical drive.