KeyCue (20 euros; ~$28) is one of those Mac applications that seems unnecessary until you really need it. I don’t recall how I heard about it but I sure am glad I did.
KeyCue is a system-wide application that displays all of the keyboard shortcuts and commands that are relevant for your context. That means it shows shortcuts for the finder when you’re in finder but shortcuts for Aperture when that application is active. You trigger KeyCue with a user definable keyboard command (my is a double CMD tap). You can either invoke a command by keyboard shortcut or by mouse click.
Now you may not be a keyboard shortcut kind of user. But that’s the beauty of KeyCue. It shows every available menu action in one on screen view. If you’re having a hard time remembering exactly where a menu item is buried, then KeyCue is a must have application. It's a little pricey, but a nice little gem when you need it.