Comprising of a virtual modular rack system with over twenty modules, built in open source some based on selected Mutable Instruments modules, it give you the opportunity to build complex patches, hook them up to MIDI devices and save. Its not a completely alien concept, Softube Modular and NI Blocks being other examples of the idea. But what makes VCV different, is that it appears to be completely free and built as an open source project. Indeed with the Mutable Instruments modules, they are also open source, hence them being available in the system.
Registration is required to download the free modules (all done from within the UI). We downloaded a copy to tinker with here (OS X 10.12.6) and can confirm that it does appear to work as advertised, even though its still only version 0.3.0. With a reasonable selection of modules, its possible to create some complex sounds - though perhaps the only real missing elements are different flavours of filters. There being just the one for now - the Fundamental VCF
However, some brief tests confirm that it is possible to create audio rate mod routings between the modules.
Its not clear what the commercial model is for this is (if any), but at zero cost, its got to be worth a try right?
Currently its standalone, but in a few weeks, they are planning to create a VST/AU plug-in bridge for the system.