I am generally not a huge fan of these type of sites. the first one was very creative, and the rest seem to be just copying, however take a look.
http://www.onebillionviews.com
I purchase a $20 ad yesterday. 36 hits from it today alone. no sales yet, but for a $20 one time ad that will last a minimum of 5 years, I don't think it was a bad return for traffic.