Windows Phone Apps that are 'featured' in the Windows Phone Marketplace often get some pretty nice results re downloads.

Now there's a contest to help you get your app featured in its own digital advertising campaign. Each month from December through February, up to seven apps will be selected and promoted in campaigns created and funded by Windows Phone.

If selected, you could get a digital ad camapign with banner ads that will run across the MSN Network. You will get one million ad impressions for your ad . A few selected apps will receivefeatured placement in the Windows Phone Marketplace. 

Course, you know there's criteria. The app must be published prior to submitting. The App must target Windows Phone 7.5 and use Fast Application Switching as well as a Live Tile. Additional criterial around originality, innovation, easy of use, and metro styling will be used to pick the winners as well.

For more information, or to enter your app, go to http://www.wpyourapphere.com  . The contest ends 12/22/2011, but there is a judging date on 11/16 as well.

I try and keep a current list of various contests I know about at my wiki - http://dfwiki.devfish.net. Enjoy.