1) Social Media - use your Facebook profile or fan page to invite your following to you site. Other social sites such as Twitter, Google +, Pinterest, and Linkedin are good as well. Socialize with your following and then casually invite them to see your offer on your site.
2) YouTube - create videos targeted towards you niche area, include your site URL in the description. Invite them to check out you site, use some small giveaway to get them to opt-in on your list.
3) Article Marketing - you can write and post articles in directories such as EzineArticles, and rank for specific keywords on Google.
Comment on these and also include other ways you use to get free traffic to your site.