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.
Sizes: Leaderboard [728x90] + Rectangle (M) [300x250] + Banner [468x60]
Google is the leader in search technology on the internet, and their AdSense Network offers accurately targeted text adverts targeted to the actual content of each page, therefore being able to offer high eCPMs.
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.
Ad Pepper is a rapidly expanding online ad network with offices all around Europe and in the USA. Established in 1999, and with this wide representation they are able to gain high paying campaigns for many geographic areas. Also, with NET 30 payout and a 50/50 revenue share they are a solid option in your ad chain.