Helm is described as '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' and is a Music Production App in the Audio & Music category. There are more than 10 alternatives to Helm for a variety of platforms, including Windows, Mac, Linux, BSD and iPad. The best alternative is Dexed, which is both free and Open Source. Other great apps like Helm are Surge Synthesizer (Free, Open Source), Vital (Free, Open Source), ZynAddSubFX (Freemium, Open Source) and Odin 2 (Free, Open Source).
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.
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 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.
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.