The greatest challenge for developers, even in the small Windows Phone 7 marketplace, is getting their apps noticed by buyers. AdDuplex has an interesting solution to this problem.
They are creating an in-app ad network where 90% of the ads are for other applications, with developers helping other developers sell their apps. A certain percentage of the ads are traditional ones, to support the service.
The service is currently in closed beta, but one can request an invite here.