According to a recent study by All About Windows Phone, Microsoft’s Windows Phone Marketplace now has over 40,000 apps , this is up from 35,000 applications in October.
It looks like over 10,000 new apps have been added within the last 90 days, and there are now over 10,000 developers creating applications for the Windows Phone Marketplace.
Microsoft still has a fair bit of work to do with their app store if they want to catch up with Google’s Android Marketplace and Apple’s iOS App Store.
Source All About Windows Phone