Yes SEO is the main source of 'free' reoccurring traffic, paid advertising gets you a single visit with no additional return on the investment. There are free social sites: Facebook, Pinterest, etc. where you can setup assets that attract prospects.

Here's a Search Engine Optimization promer by an SEO authority, too much of the stuff taught by others is out of date or simply inaccurate.

http://www.seomoz.org/beginners-guide-to...

SEO is just a catch-all for having a strategic vision on how to drive traffic. As the Oatmeal argues correctly, the best way to drive traffic to your site is create awesome content people like and share. SEO consultants seem to think they can turn crap into gold. They can't. Gold is gold. Crap is crap.

Use social media. It is a great way to get recurring traffic. Get a google+ profile and other social media accounts such as twitter, etc. Then start interacting with people. Leave resourceful comments and share other people's sites. You will start seeing traffic.

OTher options are google adwords,Content marketing,Directorysubmission etc.These are offpage seo techniques.

My website (http://www.thatexists.com) is my first amateur attempt at a content website. I didn't pay particular attention to SEO seeing as I didn't know very much about it. My question is: now that it's well underway, is there any other source of recurring traffic other than through SEO?

Any answers/suggestions would be welcome!