Voltage Modular is described as 'cross-platform, modular synthesizer for Windows and OSX in AU, VST, VST3, and Pro Tools AAX formats that also runs standalone on both platforms'. There are more than 10 alternatives to Voltage Modular for a variety of platforms, including Windows, Mac, Linux, Android and iPhone. The best alternative is VCV Rack, which is both free and Open Source. Other great apps like Voltage Modular are Cabbage Studio (Free, Open Source), miRack (Paid), NimSynth (Free, Open Source) and SoloRack (Freemium).
VCV Rack is a modular synthesizer inspired by Eurorack systems. It acts a host for virtual modules which perform various audio processes. E.G. oscillators, filters, envelope generators, sequencers and more that can be connected freely in almost limitless combinations.
SoloRack is a modular synthesizer that mimics the usability and functionality of hardware Eurorack modular systems. Whether you are an experienced Eurorack user or strictly a software user you will most probably find something new within almost all modules available.
Multiphonics CV-1 is a powerful platform for assembling and playing patches. With a simple and straightforward interface, Multiphonics is a fast and efficient creation tool for musicians and sound designers. It is also a great learning environment for newcomers.
Twist is a node-based audio synthesizer that lets you create sound effects and soundtracks. Its modular nature makes it possible to produce any kind of sound by just "connecting the dots". It's built with C++ and available for Windows and Linux desktops.