The creator of the legendary PPG wavetable synthesizer, Wolfgang Palm, has released an iPad version. The PPG WaveGenerator allows the user to create his own waveforms. This app has been out for a few months, but it is one of the best synthesizers that are currently available for the iPad. It can receive MIDI from apps like SoundPrism Pro.
The PPG WaveGenerator costs 18 euro. There is also a free version to download with limited options, the PPG Free, but be prepared that you want to buy the full version after trying it out! In the video below, the PPG Wave Generator is presented by Wolfgang Palm himself: