The iPad battery, in my opinion, is amazing, I rarely have to worry about it running out of juice. But if you are a heavy user who has battery issues, and don't mind adding some thickness to you iPad the Kensington PowerBack might just be what you need.
The PowerBack case from Kensington has a 4400mAh lithium polymer battery that should add about 5 extra hours of usage time. In addition you can use a microUSB charging port instead of the fancy Apple dock connector. But wait, there is more, you even get a kickstand. Not bad for $129.99?