Cheat Sheet for the Mac OS resolves this by providing a complete list of short cuts for virtually any open application. Simply press and hold the Command button and a "cheat sheet" appears on your screen detailing all the short cuts for that particularly app. Particularly helpful for when I'm using Adobe Lightroom or Photoshop, it also becomes a great tool for when I'm using my web browser or Mail.
The customization on this application is minimal. You can set it up to launch on start-up and change the duration that it remains up once you release the key, but that's about it. However, that is more than enough as it provides me a great way to remember a shortcut that's eluding me.
The application is free and is available through App Store.