OutBrain is an advertising network I was working with briefly in the past.
http://www.outbrain.com/
If you've ever seen those links at the bottom of a blog post showing related posts with tantalizing images that nearly all link off-site, this is probably what powered them.
They have a Wordpress widget that will serve up the ads on your blog, and you can pay to have your content shown on other blogs. The CPC is generally quite low. It's the kind of system that works great for some niches, and not at all for others.