I love Google Adsense, but I have to say my earnings there are still pretty modest. So I have decided to join Bidvertiser (but still keep Adsense on some blogs, in order to be able to compare the two). They only require you to make $10 (instead of Google Adsense's $100) in order to get paid, which is great for smaller sites and blogs who want to get started.
They also have a referral program. (see the right side bar to join through me)
Bidvertiser really seems great so far!