KeyRocket for GMail is a plug-in for the Google Chrome browser that helps you learn useful keyboard shortcuts
Taken from the Chrome Web Store description of KeyRocket for Chrome:
KeyRocket for Gmail™ is the fastest way to learn keyboard shortcuts for Gmail™ and Google Apps™ email accounts. It recommends keyboard shortcuts while your work where it matters. KeyRocket informs you, via a subtle notification, when you could have used a keyboard shortcut instead of a series of mouse clicks.
And the informational video (also borrowed from the Chrome Web Store):
Personally, I use keyboard shortcuts all the time, it means I don’t have to take my hands of the keyboard just to make one click. It can also mean that you don’t have to click through menu after menu to complete an everyday task!
Another great benefit of keyboard shortcuts is that common actions – such as saving, opening a new document, printing, cut/copy/paste etc – usually have the same keyboard shortcuts no matter what program you are using, so you don’t have to hunt a toolbar icon down! This is especially relevant to anybody who hasn’t upgraded to a Microsoft product using the “ribbon” (e.g. upgrading from Office 2003 to Office 2007)
Alternatively for quick reference you could use this cheat sheet provided by Google, or this printable version