Tribal Fusion is good if you are big
Adversal is nice if you dont mind pop unders
Extended Media is another
Snow Motion-- Research this One they may have stopped paying out.
Open X market Place gives uptions. I have never used them but will in future.
Now for the bad news payout is real low unless Customers are targeted 10 cents to 15 cents per 1000 views is average. Foreign Traffic is worse.
There is a group of companies using Right or Real Media platform Most I know have bit the dust but I have heard of a few still surviving.
Another I have not used is Project Wonder or Wonderland cant remember
I hope this helps