I didn't find any sub-categories for the categories in the iPad market, I found it only in the iPhone market
But you can see the top piad & free apps for every category
To do that, go to the iTunes store (from the left sided menu in iTunes: STORE->iTunes Store)
Then you'll see the types of stores in the grey bar at the top of the program. Point the mouse on "App Store", and you will see a drop down menu. Click it to pop out of the categories. choose any category, and u'll find the top paid and free apps for that category.
Regards,
Mazen