Everybody knows about Google Adsense (adsense.google.com). Did you know that there are other alternatives?
I've created a website where I collect Ads Services and compare them with each other. As I found out while registering with them, they may have requirements that may prevent you from using their services.
Often the type of website is a major issue. The traffic generated by your content may also be another hurdle as some providers to require more than a thousand visits per day.
Visit ads.crombz.com to view the ads providers that can be easily integrated in your website.