I only work with the twrpsar professional auto-responder. Many other good auto-responder accounts exist online but not many include such a vast array of features including:
   * mass e-mail to your opt-in clients, ideal for admin news, announcements etc
   * mlm durability feature that allows your downline to copy your mailing series at the click of a mouse, and supports password protection
   * easy capture page integration
   * automatic integration of unsubscribe link on every mail so you do not need to worry about spam complaints
   * split mailing including demographics
   * ability to ban domains as well as subscribers

Free services

you will find plenty of free resources online including free auto-responders (Listwire for instance), however they are free for a reason! Although they are ideal to begin with and to learn how to set up, the drawback is the amount of competetive advertising they create for your business, at the bottom of every mail you send there is an attached advert for the auto-responder account you are using, this can detract from your main message to your subscriber. Worse still (and I have experienced this when recieving a subscription mail from such an account), when your subscriber gets his/her confirmation mail to accept your e-mail series, they are taken to a third party advertising page: this can then put your new subscriber right in front of one of your direct competitors! Even before they begin to recieve your mails!