Keyboard shortcuts don't just save you time, they're also good for you. Using shortcuts is one of the easiest ways to wean yourself from your mouse and cut down on repetitive strain injuries in your hands and wrists. So any app like KeyCue that can help you learn new shortcuts and remember old ones is worth a look.
KeyCue is a very simple app; you just hold down the command key (or any key combination you choose), and KeyCue instantly displays a clear, clickable menu of all the shortcuts currently available in the application you're using.
KeyCue comes with many variations and preferences (everything from different menu themes to the ability to change the pop-up menu time delay), but the instant shortcut menu is its core functionality. If you also use the macro-program Keyboard Maestro, KeyCue is even more useful, as it will display the shortcuts for all your predefined macros as well. The only major strike against KeyCue is its price, which might be too high for some users given the app's narrow functionality.