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.
KeyCue - Menu shortcuts are supposed to provide quick access to frequently used menu commands.
CheatKeys - View Windows keyboard shortcuts in the current application.
cheat - Cheat allows you to create and view interactive cheatsheets on the command-line.
Hotkey Commander - Find out and override what hotkey combination is registered by which application.
Pretzel - Keyboard shortcuts for the app you're currently using
KeyCombiner - Organize, learn and instantly look up keyboard shortcuts