You can use the Google AdPlanner (wikipedia) to create virtual slots for advertising on your website, then fill these slots with whatever ads you like.
You can use AdSense, or any other Ad Network or Affiliate Network (cj.com,etc..)- or even sell slots in your website directly to companies who wish to advertise on your website.
The ad planner will help you report results of how many times ads were shown, create billing for companies who are showing the ads, and in general help you organize your inventory of slots.
If you have a very large website, a product called OpenX does about the same thing but you host it on your servers. For a small to medium sized website there is no reason to use such a monster, but you might find it useful for larger websites.