Dexed is an FM-type softsynth developed by Pascal Gauthier of Digital Suburban. It is an ideal application for importing, playing and editing/managing sysex patches from both the classic Yamaha DX7 synthesizer and the TX7.
Surge is an open source digital synthesizer originally written and sold as a commercial product by @kurasu (Claes Johanson) at Vember Audio. The synth uses subtractive hybrid synthesis and each patch contains two 'scenes' which are separate instances of the entire...
Helm is a free, cross-platform, polyphonic synthesizer with a powerful modulation system. Helm runs on GNU/Linux, Mac, and Windows as a standalone program or LV2/VST/VST3/AU plugin.
ZynAddSubFX is a fully featured open source software synthesizer capable of making a countless number of instruments, from some common heard from expensive hardware to interesting sounds that you'll boost to an amazing universe of sounds.
Omnisphere is the flagship synthesizer of Spectrasonics® - an instrument of extraordinary power and versatility. This award-winning plugin brings many different types of synthesis together into one amazing-sounding instrument that will inspire a lifetime of exploration.
A virtual analog synthesizer, Tunefish v4 was developed as a smaller replacement of Tunefish v3 with roughly the same power - some might argue it's even more powerful. It was also ported over to Linux and MacOS and was given a new polished UI.