See Tabletop and other music-related apps here: http://www.sweetwater.com/shop/ios/apps.php
Mitch takes an in-depth look at Tabletop, a music production environment for your iOS device. Its modular design lets you combine different modules like drum machines, synths, and effects.
In all, the free version of Tabletop comes with 12 modules to start with, and more available for in-app purchase. Third party manufacturers like Akai and Arturia are making modules for use with Tabletop too!