CPM media with frequency cap specifically these guys are about the only ones who have this that I can think of. I use them on occasion myself and traffic converts pretty good if you know what you're doing. They also have pixel tracking so you can track whatever media you buy and see how it converts.
Form what I see they have 68mil US impressions daily and 26mil UK impressions on 300x250 banners and 1.7mil US popunders
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