Yoshimi is an algorithmic MIDI software synthesizer, originally forked
from ZynAddSubFX.  It synthesizes in real time, can run polyphonic or
monophonic, with multiple simultaneous patches on one or more MIDI
channels, and has broad microtonal capability.  It includes extensive
addititive, subtractive, and pad synth capabilities which can be run
simultaneously within the same patch.  It also has eight audio effects
modules.

WWW: http://yoshimi.github.io/