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.
cheat - Cheat allows you to create and view interactive cheatsheets on the command-line.
KeyCue - Menu shortcuts are supposed to provide quick access to frequently used menu commands.
cheat.sh - The only cheat sheet you need Unified access to the best community driven documentation
CheatKeys - View Windows keyboard shortcuts in the current application.
explainshell - Match command-line arguments to their help.
Pretzel - Keyboard shortcuts for the app you're currently using