My personal recommendation is
oxado.
Oxado is like adsense, it finds the best ads by scanning the contents of your site. For some other networks, you have to write keywords for every site you register.
It uses Miva amongst other advertising networks, and guarantees the best payout in the industry.
They also have a GREAT support, I contacted them and 10 minutes later I had got an answer!
The downside is that they pay in euro, so if you're living outside EU you have to cashout with moneybookers.