Inblogads.com

So here are the requirements :

1. A WordPress blog hosted on your server (hosting service) so in blog ads don’t accept websites on a shared hosting like wordpress.com or blogger.com
2. Google PR(that is a vital part. You will get more money if you have higher PageRank, as far as advertisers would pay you for an extra high PR link)
3. Descent traffic (another important part, they might be paying on CPM (cost per 1000 pageviews) basis, so the more traffic you have, the better)