Simple and handy overview of all shortcuts for any application. Easy to use: just press ⌘ twice and hold to see the list.
KeyClu's answer:
The CheatSheet becomes not available, so here its successor.
Pretzel - Keyboard shortcuts for the app you're currently using
KeyCue - Menu shortcuts are supposed to provide quick access to frequently used menu commands.
CheatKeys - View Windows keyboard shortcuts in the current application.
Hotkey Commander - Find out and override what hotkey combination is registered by which application.
cheat - Cheat allows you to create and view interactive cheatsheets on the command-line.
Alfred - Alfred is an award-winning app for macOS which boosts your efficiency with hotkeys, keywords, text expansion and more. Search your Mac and the web, and be more productive with custom actions to control your Mac.