A lead magnet (a.k.a. an optin bribe) is something amazing that you give away for free in exchange for an email address. This doesn’t have to cost you anything to create; most lead magnets are digital materials like PDFs, MP3 audio files, or videos that you can create yourself at minimal or no cost. It can be absolutely anything you want, so long as it provides value to your visitors for free.
One thing that one must not forget is also integration with your existing website. If your running on Joomla, WordPress, Drupal or such likes, better you find how the user integration will go with your CMS and one of those email services. You don’t want the client to have to register for your autoresponder on one form and then have to register a second time for your website! You may find that plug-ins to do such things might restrict your choice, check it out first or consult with your webmaster!