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.
VirtualMIDISynth - VirtualMIDISynth is a software MIDI synthesizer implemented as a Windows multimedia user driver...
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.
ZynAddSubFX - ZynAddSubFX is an open source software synthesizer for Linux, Mac OS X and Microsoft Windows.