I like NameDrive the best

... Seren wrote that Dynadot had a parking page with ads. Most registrars do this, but honestly any of the companies dedicated specifically to parking are going to give you a much better PPC and revenue share than will a registrar. The simple reason being that the registrars make their money selling domains, so they don't need to try to impress you or keep you on by providing you with a great PPC.
Cheers,
Lucas