Clicksor
operates in a similar fashion to Google AdSense (see below), but draws from a more limited selection of advertisers. Clicksor is operated by YesUp, the company that has - for years - supported the PayPopup popunder program. Payment is made fortnightly on a NET15 basis once the $50 minimum threshold has been exceeded. Worth exploring.
UPDATE (2 November, 2004): Clicksor now supports image-based contextual ads in its leaderboard and banner creative sizes. These units are priced on either a CPM or CPC basis.
UPDATE (4 December, 2004): Clicksor now offers the ability for publishers to optionally launch CPM-based popunders from their contextual code. This sets the program apart from Google's AdSense.
UPDATE (30 January, 2005): The minimum payout when made via PayPal has been lowered to $20. Publishers may also opt to show their links on interstitial-style creatives.
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.