I placed Google adsense on this website. I'm trying to earn some money so I could pay off the hosting of this website. Please, if you read this, click on one of the adds on your right.
The system works really easy. It is very easy to implement this on every website. If you have a site or blog with several visitors per day, it could be a nice extra and it wont cost you a dime!
To check your account they deposit a smaal amout, in my case it was like 19 euro cents. Then when your earned money reaches 100$ they will deposit it on your bank account. I never had this, but you could help me get this ;-)
Try it out yourself, the only requirement is a free google account!