Keyboard shortcuts have become widely adopted by many major web services, but you might not know about them. Luckily for you, there's a quick way to check.
Many of these keyboard shortcuts follow a casual standard. J and K, for instance, usually show you the next or previous post in a series. Some of the more esoteric ones, however, might need to be pointed out for you — and that's exactly what the "?" key will do in most cases. (The cheat sheet for Gmail, above, is practically encyclopedic, and you can customize them with Labs' "custom keyboard shortcuts.")
Start hitting the question mark key on your favorite websites, and you'll often be pleasantly surprised! It seems like all of Google's web services offer keyboard shortcuts; Twitter, Facebook, and Feedly do, as well. Tumblr offers some hotkeys, but strangely, the question mark key does not unveil their secrets. If I've missed any other big ones, sound off!