ZynAddSubFX is described as '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' and is an app in the Audio & Music category. There are more than 25 alternatives to ZynAddSubFX for a variety of platforms, including Windows, Mac, Linux, BSD and iPad. The best alternative is Surge Synthesizer, which is both free and Open Source. Other great apps like ZynAddSubFX are Helm (Free, Open Source), Vital (Free, Open Source), Dexed (Free, Open Source) and Qsynth (Free, Open Source).
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.
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.
Qsynth is a simple Qt4 application wrapper for the FluidSynth software MIDI sequencer for Linux. Qsynth is a fluidsynth GUI front-end application written in C++ around the Qt4 toolkit using Qt Designer.
Synth1 is a software synthesizer modeled on the Clavia Nord Lead 2 Red Synth. It combines the common sound synthesis method of Subtractive & FM synthesis. Features: • 2 Oscillators, FM modulation, ring modulation, sync, modulation envelope.
Yoshimi is a software synthesizer for Linux based on the 2.4.0 release of ZynAddSubFX, written by Nasca Octavian Paul. Yoshimi delivers the same synth capabilities, along with very good Jack and Alsa midi/audio functionality.
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.