TLDR pages - The TLDR pages are a community effort to simplify the beloved man pages with practical examples.
KeyCue - Menu shortcuts are supposed to provide quick access to frequently used menu commands.
cheat - Cheat allows you to create and view interactive cheatsheets on the command-line.
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