NAKANISYNTH looks like it might be fun to play with, and it’s free.
NAKANISYNTH is a synthesizer application that allows users to produce sound loops by freehand drawing sound waves and envelope curves. Since the only input required involves drawing two waveforms, users can easily produce various sounds intuitively without complex manipulation. This application has a keyboard where users can edit waveforms and make sounds simultaneously. Therefore, it is easy for the user to understand the relationship between a waveform and the sound it produces.