What has astonished me is that there's been no serious competitor to AdSense over the past few years.
Well, maybe that's going to change! Yahoo has now entered the field, very tentatively, and restricting itself to website publishers in the USA.
A recent article on my Marketing Magic website by Merle (she doesn't admit to a second name!) takes a look at what Yahoo!'s Publisher Network ("YPN") is offering to website owners. She goes into a lot of detail. So, if you're looking for an alternative to AdSense..