The MSP synth was my first attempt at creating a reasonably complex piece of hardware that generates sound in some way or another. It started out as a generic board containing a microcontroller, a DAC and a MIDI input interface, built so I could play around with something physical while learning DSP basics. Over the next few years it gradually evolved into a (somewhat) full-featured audio synthesizer with more features than I expected to be able to fit into a microcontroller of this caliber.
The software for the MSP synth gradually evolved, starting out as an audio sample playback device with minimal MIDI support, then gaining oscillators, a filter, envelopes, and eventually a decent set of synthesizer features including a monosynth mode, FM synthesis and even an attempt at a supersaw mode.
It was actually quite enjoyable to develop this project and I learned more than I expected to in the process. Eventually I want to port the existing software to PC, add some of the missing features I wasn't able to fit into the limited resources on the microcontroller, and eventually create a Cortex M4-based version with far better sound quality and more features.