The Apple Ipad can be used as and external sound module by anyone that has a Keyboard / Organ or even just a Midi Keyboard with a Midi connection
Korg are leading the field at the moment with sound module apps which cost a fraction of the cost of the original Keyboard Synthesiser or hardware moduel
So if you have an Ipad and would like to add up to 3300 extra voices for you music production here's a video showing just one of the available apps
Brian007