Qsynth - Qsynth is a simple Qt4 application wrapper for the ...
Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
OpenMPT - OpenMPT is a popular tracker software for Windows.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
MilkyTracker - MilkyTracker is an open source, multi-platform music application for creating .MOD and .
Vital - Vital is a spectral warping wavetable synthesizer with drag'n'drop modulation workflow and animated preview of the synth's inner workings where needed. Comes with many modulation sources (including audio-rate), MPE support and FX chain.