VirtualMIDISynth - VirtualMIDISynth is a software MIDI synthesizer implemented as a Windows multimedia user driver...
Zebralette - u-he makes creative software synthesizers and effects.
ZynAddSubFX - ZynAddSubFX is an open source software synthesizer for Linux, Mac OS X and Microsoft Windows.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Phasex - PHASEX is an experimental software synthesizer for use with Linux/ALSA/JACK.
TiMidity++ - TiMidity++ is an open source MIDI to WAVE converter and player.