Google Adsense
Google Adsense is googles ad providing program. They have multiple sizes for your ads and provide text based, and image based ads!
Adbrite
Adbrite is a lot like Google Adsense in the way it provides ads. They provide ads in multiple sizes.
Bidvertiser
Provide text based ads that are customizable to fit your site.
Yahoo Publisher Network
Yahoo Publisher Network provides ads in much of the same way as Google Adsense
Kontera
Kontera provides in-content ads which are great for blogs. They also have plugins for blogs making integration very easy! The plugins also help you choose which pages to display ads on!
