I bet a lot will say Social Media

Simple answer (for me): Youtube.

- Tom

For me it's been forum marketing. Youtube can work if you know how to market it (and have the passion to do it). In the past article marketing with Ezinearticles was all the rave. Now all i do is post on my blog, and do forum marketing.... and some email marketing.

Social Media and Forum traffic.

email marketing

I use free traffic all the time to test my landing pages. I like to shoot for at least a 30% opt in rate before I invest in paid traffic.

Youtube works ok but tends to bring down my epc's on the backend.

There are a lot of good free networks out there it just takes being consistent and putting in the time.

Facebook and twitter work well but again like facebook your doing with a social network site.

Doing email swaps can work if you have an established list that opens your emails.

Thing you want to do is sure use your free sources and build your list and you will make a commission on the backend and when you do you re invest in some ppc or banner traffic. Test and optimize your campaigns and your epc' can be a lot higher.

Somebody mentioned forum's and yeah I forgot to mention that.

Video distribution on YouTube, DailyMotion, Vimeo and other video sharing sites.

Social Media such as Google +, Twitter, FB, etc.

Free classified ads on CL, Backpage, USFreeAds, Classifiedads(dot)com, etc.

Forum posting.

Mutual links with blog owners in a similar niche (not the same niche) as my blogs.

Facebook has made me multiple 5 figure deals! Reddit is also a big one!