BoltMail
The Free Bonus Hook-In
Free is overused these days, especially on the Internet. However, if you're looking to grow your subscriber list, then create or source a product of value to your visitors (such as an eBook or discount coupon) and offer it to them for free when they signup for to receive your newsletter.

To make sure they don't simply type any email address into your subscription form, setup an autoresponder to send them the free bonus 1 hour after they subscribe.