As affiliates you must always be on the lookout for new traffic sources.
In previous posts I showed how to use google adplanner / quantcast to do this.
I found a new way today which has a lot of potential.
I stumbled on this site, when I was reading this excellent article about pinterest seo & marketing.
They showed this diagram:

Which shows in which industries the Pin It button is most used.
The thing that sparked my interest was this:

What does Built With do?
BuiltWith tracks the technologies used by the top 1m sites on the internet, and shares with you the % of sites using them and whether its growing/declining.
For example:
13% of the top 10k sites use the Facebook SDK
20% have the Quantcast audience measurement plugins installed
13% run on NGinx 
It also shows which technologies are growing the fastest:
Jquery usage by the top 1m sites has grown by 0.12% in the last week
Google Adsense for domains by 0.05%
So why is this relevant for finding traffic sources?

What it shows you is the top advertising technologies found on the top 1m sites, and its growth by month, 6month and year!
http://trends.builtwith.com/ads
For example:

The one you want to pay most attention to though is the Advertising Growth tab.

Or check the 6 month list.
Then just pick a advertising solution thats relevant to what you do, and inspect the detailed stats page:

Finding related traffic sources
Or, lets say you have a campaign running profitably on AdSonar and you want to find similar networks?
Go to the profile page:
http://trends.builtwith.com/ads/AdSonar
And check out the Similar Coverage list:

Scaling a target
Or if you are doing really well on a particular site on the google content network, find out what other advertising solutions that site uses!

http://trends.builtwith.com/topsites/Google-Adsense
And view the profile of that site:

http://builtwith.com/wunderground.com
Have fun!
Amazing tool.
Nice find Tijn!
I'm gonna call you a gold digger Tijn! This site is so useful. Thanks for sharing!
they have a chrome plug in been using this for a while.... nice mention
This is some pretty powerful information. Whats growing is working
Hey could somebody put together a list of advertising platforms and what is required upfront? Maybe based on the top places form that list?
Awesome info Tijn. Need to spend some time on this for sure.
Very nice find! Thanks for the referral!
Very useful and actionable. Thanks tijn.
Came across this gem and thought I'd give it a bump! @twinaxe. 
Amy
Nice content for affiliates!