There are now more than 40,000 apps on Windows Phone Marketplace, according to a report released last week by tech blog All About Windows Phone.

The publication based its findings on its own tracking system. About 165 apps and games are being added each day, and the application storefront is forecast to hit the 50,000 mark by mid-January.

About 4,770 apps were added in the last 30 days. Of those, 85 percent were apps and 15 percent were games. Approximately 68 percent were free, 23 percent were paid and the remainder were paid but came with a free trial offer.

The publication warned that the number of published apps differed from the items actually available for download in a given market. For instance, there are 33,950 Windows Phone apps available for download in the United States but just 30,265 available in Australia.

Momentum for Microsoft’s operating system appears to be growing ahead of Nokia’s commercial release of its new Windows Phone 7 devices early next year. Microsoft’s Mango update for the platform has also helped spur developer interest.

However, the app offerings for Windows Phone are paltry compared to the selection on Android Market and Apple’s App Store. There are more than 500,000 apps available for iPhone and iPad users, and more than 200,000 apps for Android-based smartphones and tablets.