Domain Sponsor has been highly praised by most domain professionals. The service is monetized by Google and it a part of Oversee.net - the company behind the DomainFest conferences.  Their system "automatically optimizes" your domains. Domain Sponsor sends out checks quickly and reliably. PayPal payments are now available too.

Domain Sponsor was one of the first PPC parking services that moved to a hybrid pay-per-click / pay-for-performance model. They no longer pay the same amount per click on a keyword - rather they attempt to analyze the "quality" of your traffic. This appears to mean that they want to know if visitors really buy stuff from the sites that they click through to. Now almost everyone does this.

Domain Sponsor allows you to create a site with custom graphics. You can also set whether the domain rotates between different landing pages or gives people the same page every time.

Domain Sponsor's "auto optimization" process starts with semantically constructed generic search page that keeps track of what people search for and click on. These links eventually migrate to the main page. The process works quickest for high-traffic domains. You also have the ability to add or remove keywords. The have upgraded their processes several times. It is now possible to get graphic-rich pages that will rotate with the more generic pages for the system to see which pages earn more revenue.

The stats that I get from Domain Sponsor sometimes show fewer visitors than other services show for the same domain. Payouts are competitive, however. Digital-Daycare.com is an example of a Domain Sponsor domain.

Domain Sponsor is highly recommended.