Building an email list is as important as producing content because then you own the reader and have a personal connection to it. Twitter followers, Facebook fans are not that active in clicking your links, commenting or buying your products. Having an email of your user you have possibility to email him or her at any time and most likely get a response.

If you don't want to spend any money on your email list start with Mailchimp which gives you a free account for up to 2,000 subscribers.

AWeber - Email marketing and autoresponder software that's easy to use & only $1 to try.

Mailchimp - I am currently using Mailchimp. Used for collecting emails and sending out the weekly newsletter.