The Android Marketplace, while far behind the iPhone App Store, is streets ahead of the competition from other platforms, and has now hit the milestone of 20,000 available apps. For comparison, there are approximately 100,000 on the iPhone platform.
The figures aren't totally official - Google doesn't release stats on the popularity of the store, but AndroLib is a site that has tracked the Android Market since it opened and so is about the closest that we're likely to get to an official figure unless Google has a change of heart.
Of those apps, 62.3% are free, approximately 100 are added every day, and 44.6% of reviews of those apps rate them 5 stars out of 5. That's some pretty serious satisfaction going on there. Only 17% of reviews award 1/5 stars.
The rate at which apps are being produced is slowly growing, topping 3500 in November. More have been produced in the first half of this month than were produced in the entirety of July 2009. It'll be interesting to see how the figure is affected by the Christmas break however.