Adsense by Google.
One of the easiest affiliate programs out there I think is Google's Adsense. Signing up is easy and as I have always said it makes no sense to not put some kind of affiliate program on your blog to make some passive income. It will be slow at first but as readership grows you're response to the google adsense adds will grow and your income will follow. There is a button at the right column that will bring you directly to the Adsense page. It's as easy as one, two, three.