Fermion, based on the open source - Mobilsynth, is a virtual analog synth for the iPad.

Fermion was designed to act like a vintage hardware analog synthesizer. It used the size of the iPad to its advantage and made sure that all synthesis parameters are visible along with the keyboard. You can then tweak your synthesis and sounds while playing because all the parameters are within reach. This will give you more freedom in adjusting sounds while performing.

Just like the Roland TB-303, or a classic Moog, you don't just get classic synth sounds, but classic synth interface as well. The Fermion features monophonic synthesis, dual oscillators, Tremelo/Wave, Envelope Generator module and many other standard synth features.