Ratatap Drums for iPad
Ratatap Drums is a free beat making app for your iPad. It boasts 10 drum kits and lots of great sounds. I recommend upgrading to Ratatap Pro via in app purchase for $2, which will give you the ability to edit drum size, appearance, and location. You can also add reverb, and tuning to your drum sounds in the Pro version.
Drum Meister for iPad
Drum Meister is a drumming application for the iPad. It comes from a top-down view and gives you an instant drum kit at your finger tips. You have four kits to choose from with extra add-on sounds available for each kit. You also have the ability to move individual drum sounds around and change the size of them to maximize playability. Much like Garageband (for iPad) the drums are velocity and positionally sensitive. This means that the sound corresponds to how hard you hit that drum and where exactly you hit that drum.