Adx CPC and CPM based, has pixel tracking so you can optimize. You can see site IDs and kill traffic that doesn't convert easily. Accepts credit cards (which is big for me) also paypal.
Exoclick - can be a bit of a pain to get approved due to them trying to keep out malware scammers out, CPM only, has access to some large tubes though (prepare to compete) and also has pixel tracking. With them you can also see average bids in advance so you can pick and choose which sites to target