The site claims that books are the most popular category, making up 17% of all apps. Games are second (14%), followed by entertainment (11%) and education (7%). Music-based apps account for 4% of all apps on the store.
In related news, the Royal Pingdom site reports that 70% of the 250,000 apps on Apple's App Store are charged for. In contrast, 64% of the 95,000 apps available on Android Market are free.