The Problem with Google AdSense, is that they make you wait until you have $100 until they send you any money and they don't do paypal.
Here is a site with some alternatives to AdSense.
http://www.rosswalker.co.uk/adsense_top10/
Actually I'm probably going to put adds on my site, are AdBrite any good?
All I could see is that their sites looks awful, but it seems that they will pay you when you have less money which is good.
Another one that I've heard of from a friend is
Project Wonderful.