AdSense is my favorite, cause it bring the most money.
For sites that I can't use AdSense on (poker/gambling mainly) I use
AdToll - I like the easy to use layout of the site, and they have some pretty decent sites using both the advertising and publishing.
Depending on the site, PPP ads might be a good source of additional revenue also. They can be used with AdSense, and most other PPC programs. See my signature links for more information about pay per play.