Diagnostics Mode
Power on, press and hold D (does not work on MacBook Pro 2011)
Power on, press and hold option-D (internet)
Recovery Mode
Power on, press and hold command-R
Power on, press and hold option-command-R (internet)
Reset NVRAM
Power on, press and hold option-command-P-R for approx 20 seconds, machine will restart
Reset SMC
Hold down left shift-control-option keys, press and hold down power, let go off all 4 after 10 seconds

Using Manage Storage
Click on Apple Menu… About This Mac… Storage… Manage….
Check “Music Creation” (Can be deleted)
Check “iOS Files” (Delete old backups no longer required)
Manual
/users/name/Library/iTunes/iPhone Software Updates
/users/name/Library/iTunes/iPod Software Updates
/Users/name/Library/Application Support/MobileSync/Backup

If Artwork is missing from the Music application, or if you want to force update artwork cached from a shared library:
Quit Music, TV and Podcasts.
Move all the files from the “~/Library/Containers/com.apple.AMPArtworkAgent” folder into a backup folder elsewhere.
Log out then back in.

To enable the MacOS startup sound which was disabled in certain versions of OS X:
sudo nvram StartupMute=%00

If iCloud files are stuck on "waiting to upload”, or folders constantly have download icons:
killall bird
rm -rf ~/Library/Application\ Support/CloudDocs
killall bird

For standard load/save dialog boxes instead of OneDrive:
sudo /usr/bin/defaults write /Library/Preferences/com.microsoft.office DefaultsToLocalOpenSave -bool true