Keyboard shortcuts help us achieve greater efficiency by not having to lift our hands from the keyboard to use the mouse. The only problem is remembering them all. If you use Gmail, Google Calendar, or Google Reader, the single most important keyboard shortcut to remember is the question mark ("?").
In the Google services we mentioned above, hitting the "?" will bring up a grid of all the keyboard shortcuts that are available for that service. Clicking on the "Open in a new window" link will display the shortcuts in a new browser window so that you can save them.
Here's what they look like:
You're not likely to remember every shortcut there is, but with some practice, you should be able to remember many of them. For when you can't remember them, however, there's always the "?."