Shortcuts for Gmail is a lot like previously mentioned Eve, a Mac app that does the same in the OS. Every time you perform an action for which there's a Gmail shortcut, a pop-up alert will appear in the lower-right corner of your Chrome window, letting you know how you can do the same thing without taking your hands off the keyboard. You'll have to have shortcuts enabled in Gmail (under general settings and "keyboard shortcuts") for this extension to work.
Shortcuts for Gmail is available now in the Chrome Web Store. The publisher of the app has been validated, but be prepared for the hefty permissions it needs to work. If you'd rather learn the hard way, remember you can always press "?" in any Gmail window to bring up an overlay of shortcuts.