One of the lesser-known aspects of Gmail Labs' useful Canned Responses feature is that it's available as a filter action. This means you can auto-reply to messages that match criteria you set up with a canned response. For example, you can say that any message from Aunt Bertha with "Fwd" in the subject line should automatically get the response: "I love you Aunt Bertha, but please stop forwarding me chain letters."
As for me, I'm using this to deal with certain particularly persistent public relations people who send press releases to my personal email address. I used to just automatically filter those messages to a label I never look at; now, I can automatically reply with the following using canned responses:
Hi--Looks like you've sent me a press release. Please remove my email address from your mailing list.
Then shuttle the message off to a label I never look at. Check out the setup in the screenshot above. In the second screen of the filter creation process, check off "Send canned response" and choose the one you've set up from the list.
For more on enabling Gmail Labs and its best features, see my picks for best Labs features (including Canned Responses).