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.
Read more about this service on their site here, and remember WMPoweruser.com also offer developers opportunities to promote their apps. Contact us about that here.