Yahoo! Publisher Network
From Seo Wiki - Search Engine Optimization and Programming Languages
The Yahoo! Publisher Network (abbreviated YPN) is a beta network launched on August 2, 2005 by Yahoo!. As the service is currently in Beta, it is currently only accepting US-Based publishers; it is believed[who?] that Yahoo! will expand this when the program comes out of Beta. YPN provides cost per click contextual advertising as well as various tools and services to assist publishers in building and improving their websites.
In May 2006, YPN caused some controversy by shutting down accounts used for MySpace layout sites. The reason given was the quality of traffic was very poor.
Also in May 2006, the Yahoo! Publisher Network's blog posted an updated version of their requirements from publishers. YPN have now explicitly stated visitors must recognize advertisements as advertisements, and images/media may not be placed to induce accidental clicks.