Contextual Advertising

AdSense
The Adsense program delivers contextual ads that look like Adwords but are actually banners generated on the fly to partner sites.

GAIN Network (Claria Corporation, Gator)
GAIN Publishing distributes a variety of popular software applications and owns and operates the GAIN Network, the world's largest in-context behavioral advertising network. The GAIN Network helps keep many popular software applications and websites free in exchange for displaying online advertising based on your online surfing behavior. 

Quigo
AdSonar gives publishers an immediate way to place contextually relevant ads onto a website generating additional revenue without investing in costly infrastructure or timely sales calls.

Vibrant Media
Vibrant Media's patent-pending IntelliTXT technology creates commercial text links from keywords appearing within pages of online content.