Forum user wolke recently posted about his monophonic guitar synthesizer. This DIY project has an impressive list of features. This is a German language video that demos the cool effects. The unique sounds remind me of spaceage music.
The code for the project is posted on Github https://github.com/wolkstein/GitSynth127
The modified audio library is on Github as well https://github.com/wolkstein/Audio
Discussion about the project can be found over on the forum.