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...
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.
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.
Tyrell is the name of a project by the German online magazine Amazona.de. A reader survey and follow-on forum posts provided a pool of ideas for a low cost HARDWARE analogue synth, which Mic 'Moogulator' Irmer used to develop quite a powerful concept.