The Yahoo sponsored search (former Overture) is the major Pay-Per-Click program besides Google Adwords / Adsense and can put you on top of leading sites like Yahoo!, MSN, AltaVista and CNN.com

While it seems that Google Adwords seem to rock in terms of traffic and prices, Overture is a perfect complement to the Adwords campaign.

The have a similar content-relevant advertising technology to match your ads to the relevant content to improve conversion - called Content Match.

According to a survey I found Content Match and Adwords offer equal matching quality at least 61% think they offer same quality in matching.

Yahoo Sponsored Search have a promo where you can sign up and get a $25 credit.

Also found a nice article by Danny Sullivan comparing the two Ad Guidelines