There's also Mellowads, I've used them a bit in the past and earnings were decent.
I'm not sure if their requirements are strict or not, I got in without any problems, but things might have changed.
As far as I know, Mellowads now requires the publishers to be ranked in Alexa rank (>100k if I am correct). A lot of people were abusing the network with very low-quality websites. The change has been made to improve the quality of the service. (Advertisers aren't interested in bot traffic, etc since bots don't buy anything)