Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
Qsynth - Qsynth is a simple Qt4 application wrapper for the ...
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
OpenMPT - OpenMPT is a popular tracker software for Windows.
Omnisphere - Piano, pad and synth VST for DAW's.
MilkyTracker - MilkyTracker is an open source, multi-platform music application for creating .MOD and .