Using free tools anyone can achieve a healthy AdSense income, and if they multiply the methods by 100 then they will do very very well with AdSense earnings. Social media sites are the first free tools that I have come to mind, but there is so many of them out there how is anyone supposed to know which one returns the most AdSense earnings.
I think the top 5 are
Facebook, MySpace, Reddit, Squidoo, and Twitter.