With the iTunes App store now over a month old, I decided to look closely at data from the U.S. store over the last three weeks. While sales numbers are not publicly available, Apple publishes overall as well as category-level rankings. There are currently just over 1,800 (paid and free) applications in the App store, double what it was three weeks ago. Games is the largest category with about 500 applications (roughly 27% of all apps), up 87% from three weeks ago. Puzzles, Arcade, and Board games are the three largest Gaming subcategories:
The fastest-growing category, Education, more than tripled over the last three weeks.
The average price per paid app is around $5.50, with 94% of apps priced at $10 or less. Prices vary considerably by category with expensive apps skewing the average price in a category: a single application priced at $449 drove up the average price of Finance apps to more than $22. Excluding the top and bottom 1% priced apps, the average price of an iPhone application is about $5.20. Similarly, by removing the top and bottom priced app in each category, we get a more reasonable estimate of the average price per app within a category (click here for details).
The Book category is comprised mostly of ebooks and while there are over 150 such “apps”, it was the only category not represented in the Top 100 rankings: 
In contrast, more than 1 in 10 of all Music apps were among the Top 100 Paid Apps:
Looking beyond the Top 100 paid apps to all paid iPhone applications, the best-performing categories (in terms of popularity) are Music, Weather, Navigation, Lifestyle, and Entertainment (click here for details).
On average, app providers have slightly over one app each, with 25 (out of the close to 1,100) providers accounting for about 21% of all paid apps: 
Most providers had 0 or 1 app listed in the Top 100 Paid Applications, with the following exceptions: Hottrix, Pangea Software, Inc., Phase2 Media, telience.com, and Electronic Arts all had 2 apps in the Top 100 list. For now, the cohort of web developers who dominate the Facebook application platform have been unable to make similar inroads in the iPhone platform. Perhaps it’s time to brush up on Cocoa and Objective-C?