[Update] I can no longer recommend this resource….
I have been a member of Traffic Testers for a few years now and it is one of my favorite resources for finding great advertising resources.
What I love about the site is that they review 10 different places to advertise every single month. In their review they let you know how many unique visitors they received, how many opt-ins and/or sales that they received from the ads they ran with that particular site.
They review both free and paid traffic resources.
This allows me to save a lot of money by not running any ads or wasting any time with any of the sites they reviewed that did not produce for them.
They started their review site in December of 2005, so there are a lot of resources for you to check out.
So if you want to receive in depth test results and reviews on over 300 of the internets top traffic and advertising sources, just head on over to Traffic Testers for complete details.