Answer.im is one of useful FREE online service which works as a auto
reply answering machine for your Instant Messengers like Yahoo
messenger, Google Talk, MSN, AIM and ICQ.
This service could be
really useful to auto respond to the messages on your instant messengers
in case you are away from your computer or busy in doing some other
important work.

It works in the following way mentioned below
1. Go to answer.im and Login through your instant messaging account credentials.

2. Setup your online account status and type in your answer text and click Save Changes button.
3. Click Exit, to exit but your account will stay online, and respond to messages by your friends.

This service will auto respond to all the messages by your friends and contact list even if your computer is switched off.