I’m quite a fan of Google’s AdManager - you can read my previous posts on the subject here and here.
At the time I wrote those posts, you had to request to join, but Google have now opened up the product for everyone to use.
If you’re looking for a product to manage the ads for your site that is a) free, b) very good and c) doesn’t kill your server with the overhead, I’d thoroughly recommend you give it a try.
