Google AdSense

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.

Yahoo Publisher Network (YPN)

Yahoo Publisher Network offers a similar contextual ad program to AdSense, but aimed more specifically at a US audience with ads which although not quite as accurately targeted sometimes offers higher CPC rates and therefore eCPMs.