I love keyboard shortcuts. When learning a new app, the first thing I do is sift through its menus and absorb as many of the shortcuts listed as possible.
CheatSheet is a cool app on the Mac App Store that does one thing and does it very well. Once installed, it runs in the background (no dock or menu bar icon), and will pop up an overlay if you hold the Command key down for a moment. This overlay will show you all the keyboard shortcuts for an app at once. You can then either finish the shortcut or use your trackpad to select a command. Releasing the command key will dismiss the overlay.
You can also adjust the length of the delay for CheatSheet or quit the app from the gear icon on the overlay.
This is a great app for Mac keyboard ninjas learning a new app or for the fledgling keyboard padawan.
CheatSheet is free, so you really don’t have an excuse to not get it.