I just discovered today that you are not allowed to add Adsense to site that contains nudity of any kind, this made me think a list of contextual advert service provider including the two well know one will be a good idea. Here are the one I can think of:
Adbrite
Adsense
Adster
Clicksor
YPN
Adbrite
Adsense
Adster
Clicksor
YPN