Yahoo! Publisher
Yahoo! Publisher is to become the advertising wing of Yahoo! and it will probably become the #1 direct competitor of Google AdSense once it gets fully working and available worldwide – maybe in one or two months. Webmasters will be able to generate revenue by displaying content targeted ads - an advertising concept that has been running for a couple of years now every since Google AdSense was launched. The site mentions “You’ll earn revenue from qualified clicks”, so they probably have an extensive anti-cheat system.
Currently, Yahoo! Publisher is in public BETA mode. The only restriction to try it is being a resident of the U.S. Too bad for me, I can’t try it for the moment… However, I was able to spot some Yahoo! Publisher ads on a couple of web sites, they are similar to Google ads: text-based with a blue border, an ad format that is becoming standardized.
When searching, I also found Google vs. Yahoo! Interface Design, a nice article comparing all the features of Yahoo! and Google!