If you have website with original content and more visitors then google adsense is a good earning opportunity. I have tried google adsense in past but I don't know their policies at that time but I have got earning from their ads. I have stopped posting on that site because of time constraint. Some of my friends getting good income from google adsense.
Getting approval for adsense is difficult task. I have got approval from other revenue sharing site and used that adsense id for my site. Google don't allow copy paste content, adult content. We have to follow their policies. I have tried some other adnetwork sites and placed their ads in my sites but their earnings are very low when compared with google adsense. Google adsense minimum payout is 100$.