UK iPhone app developer Ustwo has released what it calls "the world's most in-depth, intuitive app discovery and performance tracking tool.”
The app tracks the App Store's Top 300 apps chart from every country across the globe, and allows users to drill down into that data however they like. It pulls in data from Apple's public RSS feeds to do this.
You can browse by country, genre, position change, app name, free and paid, and by day, week or month, while setting certain apps as 'favourites'.
For a developer, that means you can, for example, set your own apps and those of your rivals as favourites, then monitor their chart performance dynamically from your iPhone. Plus there's six months of historical data to dig into.