These guys have a really nice app built around the data from the iPhone AppStore. Since I am working on iPhone apps, it's a big help to see what's happening.
I'm hoping they take my suggestion to start showing a set of price charts. It would be fascinating to see the rise and fall of prices in the various categories. Probably a lot more fun than watching the stock market these days.
Other useful sites:
MacRumor's iPhone Site - these are the guys behind AppShopper as well.