Contextual Advertising Programs

Google AdSense single-handedly revolutionised the online advertising scene in June 2003 with the public release of its contextually-responsive search-based advertising network. The system taps into three of Google's major strengths: it's accurate search algorithm, its massive portfolio of keyword-based advertisers and its powerful brand in the realm of web publishing and online marketing. AdSense allows publishers to place ad creatives of verying sizes, while the AdSense system automatically targets the text or graphical content in each ad unit to content already existing on the page. Pages that contain a review of Casablanca, then, will automatically attract ads regarding classic films of its ilk. Google AdSense operates under a cost-per-click payment model, with payments sent on a NET30 basis in accorsance with a $100 minimum payment threshold. Highly recommended.
UPDATE (13 May, 2004): Google has introduced support for image-based ads. Publishers may opt to have their existing ad code display image ads as well as text ads (or remain opted out by default), should they wish to take advantage of this new feature.