GMail gets auto-replies and canned responses

Gmail already lets you create filters based on a combination of keywords, sender, recipients, and more in your incoming messages. Turn on Canned Responses in Labs, and you can set a filter to grab one of your saved responses, create an automated reply, and hit the Send button for you.

You can set up different automated messages for different keywords, just like you said you wanted. (We're friends, so I trust you to use this power responsibly.)

How to Enable it:

1. Click on Settings in the top right corner of Gmail
2. Click on the Labs setting
3. Enable Canned Responses
4. When you are composing an email a Canned Response option will appear. This lets you save the email as a canned response.
5. For auto-replies you would setup a filter (under the account settings) using the saved canned response.
   

Link:
http://gmailblog.blogspot.com/2008/10/