VCV Rack is an open-source Eurorack emulation environment. Developer Andrew Belt has built a system to simulate interactions between 0-5 volt signals and various circuits. He’s paired this system with a UI that mimics conventions of Eurorack use. Third-party developers are armed with an API and a strong community.
VCV Rack is open-source, and the core software is free to download and use. The VCV Rack website also features several sets of modules as expansions, many of which are free. The most notable cost-free VCV offering is a near complete set of Mutable Instruments modules, under the name Audible. Beyond the modules distributed by developer Andrew Belt, there’s an ecosystem of several dozen developers, all working on building and supporting their own sets of tools – the vast majority of these are free as well, as of the time of this writing.
“Modular is for everybody”, and VCV Rack is modular for everybody.