AdSense
Despite not using it here at ProBlogger any more (here’s why) I continue to use AdSense with amazing effect on my other blogs. I have them all set to show image and text based ads and find that 250×300 pixel ads work best (usually with a blended design). I don’t have much luck with their ‘referrals’ program but their normal ads work a treat and continue to be the biggest earner for me
