Conversant (formerly ValueClick Media) currently represent the pick of the independent online ad networks due to their great support and service, as well as decent payouts.
Model: CPM; Sizes: Pop-Under + Banner [468x60]
Tribal Fusion have emerged as one of the leaders in internet advertising by only representing high quality sites with original content, and so are very difficult to be accepted by, although they do offer the highest paying campaigns and a great service.
Burst Media are one of the longest established online advertising network, being established in 1995, and have survived due to their high quality network of publishers and advertisers, and the great targeted ad buys which they are able to get for independent sites.
CPX Interactive have emerged as a great second tier network due to offering all the popular creatives with decent payouts, great support, and 100% sellout rates.