Gmail Gets "Canned Responses"
It's called "Canned Responses." To access it, go into your Settings, then Labs, and scroll down a little until you see the above, and enable it.
Once enabled, as you see above, you get a new option when composing email. You can save the email body you are currently typing as a "Canned Response," that can be selected when emailing someone.
Or, even better, you can create a filter that might send back a "Canned Response" to someone based on a set of parameters. We're assuming you might have played with Gmail's filter options (explained here, if you haven't).
Think of it as an advanced version of "auto-reply." Now most people (with the exception, perhaps, of some politicians) don't use webmail for work, but if you did, you could have a "Canned Response" that said "I'll start working on it" if your boss sends you an email.
Yes, we're sure there are more legitimate uses, and this is actually a pretty cool feature, particularly when coupled with Gmail's filter options.