Pretzel - Keyboard shortcuts for the app you're currently using
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.
explainshell - Match command-line arguments to their help.
Hotkey Commander - Find out and override what hotkey combination is registered by which application.
CheatKeys - View Windows keyboard shortcuts in the current application.