I found a cool site:
http://www.adgridwork.com
Its kind of like googles adwords and adsense, but its free and doesn't pay. But it appears to be great! You sign up, display the ads on your page, and then your ads will appear on other peoples pages too...for free. I just signed up 10 minutes ago and my ads have already been displayed 3 times. While no one has clicked it yet... i still find that to be a great deal of impressions in a short period of time!