For those who are interested in keeping app scores… Windows Phone 7 Marketplace has just crossed the 15,000 app milestone. It was only 6-weeks ago that the 10,000 app mark was reached so progress is happening. There are some high profile apps coming out within the next few months that will help increase its popularity: Skype, Spotify, Layar, and Doodle Jump Plants vs. Zombies (by the end o the year) and Angry Birds is pegged for a May 25th release date. In addition, the real jump for the Windows Phone Marketplace will happen when the mobile OS hops onto Nokia devices possibly later this year of early 2012.
Source: Windows Phone App List