If you commonly type the same reply and/or email, now you can compose your reply once and save the message text with the "Canned responses" button. Later, you can open that same message and send it again and again.
To turn on the lab, click Settings, then the Labs tab, then search for the lab (if it is not enabled, it will not be at the top - they are listed alphabetically):
Once it is turned on, you will see it as an option when composing a message by clicking the down arrow for "More options":
Compose the message you wish to save as a canned response, then click Save on the Canned Responses menu. It will ask you to name it. Once it is saved, in the future, you can click Insert on the same menu and the text will be inserted into a blank email.
We do recommend you save a canned response without a signature, since a signature will be automatically inserted.
Another way to use this functionality is to create filters based on a combination of keywords, sender, recipients, and more in your incoming messages. Combining this with Canned Responses, you can set a filter to grab one of your saved responses, create an automated reply, and hit the Send button for you.
It doesn't get any easier than that!