Domain Spa was one of the first domain monetization services. They recently re-branded themselves as Domain Parking Media.  They are creating interesting sites that do not resemble traditional parked domains.   
The most recent version of the service is friendly and flexible - and it has some great landing page templates. It also includes some features that other services don't offer.

You can set up your domains within certain categories when you first add them to the system. A generic lander shows up for a few hours, but fairly quickly you will see a custom template. DomainParking now creates portals that you can customize with their "Vortalizer" tecnnology.

DomainParking appears to monetize domains through a number of different partners, including Ask.com, Enhance, ePilot, Kanoodle, Looksmart, Miva, MyGeek, SearchFeed and Yahoo. Advertisers can also bid directly on individual templates for highly targeted revenue. Examples of DomainParking domains include MyDentalLab.com, PsychotherapyOffice.com and HowHi.com.