macOS

Change default screenshot save location

defaults write com.apple.screencapture location ${LOCATION} && killall SystemUIServer

Change key repeat rates

defaults write -g InitialKeyRepeat -int 20
defaults write -g KeyRepeat -int 6

Be careful not to set InitialKeyRepeat too low, or you may not be able to type your password correctly.

Create keyboard shortcuts for any app

In system preferences under “Keyboard”, shortcuts can be made for any app actions that are available as a menu item. Defining the shortcut requires copying the exact name of the menu item. I’m unsure how this system resolves when there are two identical menu items in different sub-menus.