Dock to corner:
defaults write com.apple.dock pinning -string end killall Dock

Enable 2D dock:
defaults write com.apple.dock no-glass -bool true
To hide the files change true to false

Undo delay dock:
defaults write com.apple.Dock autohide-delay -float 0 && 
killall Dock

Widgets on Dashboard:
defaults write com.apple.dashboard devmode YES

Now, log out and log back in. Open up your Dashboard, grab the widget you want by clicking and holding it with your mouse, and hit F12 to drop it onto your desktop. If the widgets don't appear, hop back into Terminal and type: 
killall Dock

Show hidden files:
defaults write com.apple.finder AppleShowAllFiles true 
killall Dock
To hide the files change true to false

Single application mode:

defaults write com.apple.dock single-app -bool true 
killall Dock

Copy Text from Quick Look:
defaults write com.apple.finder QLEnableTextSelection -bool true 
killall Finder

Popup notifications iTunes:
defaults write com.apple.dock itunes-notifications -bool TRUE 
killall Dock

Disable notifications iTunes:
defaults delete com.apple.dock itunes-notifications

Enable debug menu in Address Book:
defaults write com.apple.addressbook ABShowDebugMenu -bool true

Enable debug menu Disk Utility:
defaults write com.apple.DiskUtility DUDebugMenuEnabled -bool true
+
defaults write com.apple.DiskUtility advanced-image-options -bool true


Disable startup chime:
sudo nvram SystemAudioVolume=%80

Prevent Sleep:
caffeinate -t 3600  (t = time in seconds)

Disable Window Animations:
defaults write NSGlobalDomain NSAutomaticWindowAnimationsEnabled -bool false

Disable notification center:
launchctl unload -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist killall NotificationCenter

Stop help from floating over windows:
defaults write com.apple.helpviewer DevMode -bool true

 

Source:
http://lifehacker.com/the-best-hidden-settings-you-can-unlock-with-os-xs-ter-1476627111

 

defaults write com.apple.mail ABShowDebugMenu -bool true