Huge network that bought fastclick. Amazing power and properties for media buying.
http://www.valueclick.com

I dig this company and the CEO. Fun to deal with and produce quality results.
http://www.adbrite.com

-Text link based system -Flat monthly fee based on thier own algorithm -Has thousands of niche websites to choose from -50% net revenue
http://www.text-link-ads.com/

-Banner ad based I believe -Over 4000 websites -Behavioral targeting -Content is always monitored providing trustworthy ad placement -Profile based advertising like Facebook ads
http://tacoda.com/

-Rss ad based providing text, banner and rich media delivery -Uses very well-known feeds to deliver it's ads -Ads are delivered to the likes of FeedBurner -Your RSS ad stream is updated hourly providing excellent dynamics
http://pheedo.com/

-Very selective with publishers ensuring high relevance and trust -Ads are dynamically optimized and re-targeted for highest results -Cost-per-click or cost-per-conversion options
http://www.tribalfusion.com/

http://www.specificmedia.com/

-Special benefit of being affiliated with Yahoo -Focuses on the 250 best know household name websites
http://www.bluelithium.com

http://www.247realmedia.com/

http://www.advertising.com

http://www.undertone-inc.com/