I just tested the Sonigen Modular which is a free modular synth (VSTi) by Chris Jones and this thing sounds great! Its a very versatile synth and can produce really cool pads, basses, leads, etc.
Here’s some details about it (quote from Sonigen website):
Sonigen Modular is a modular software synthesizer which combines the freedom and flexibility of classic modular synthesis with the benefits of a modern software implementation. The user interface is quick and intuitive to use while still affording the sound designer great freedom. The audio processing algorithms are of high quality while still being CPU efficient.
Sonigen Modular is fully modular, with as many modules and patch cables as you want.
It combines polyphonic voice and static effect processing in one window. That means you can have multiple midi notes playing multiple voices, mix them all down to a single stereo stream and route that through a modular effect chain.
It is fast and intuitive to use, almost everything is done in the main module window. There’s no separate schematic / GUI views, no macro modules, no separate FX window. You can see what is going on just by looking at the main window.
Great sound quality. A huge amount of time has been put into developing high quality algorithms. It has very good aliasing rejection in most cases.
It is CPU efficient. This doesn’t mean corners have been cut. It means hand tuned assembly and run time optimization. The audio processing code is dynamically assembled and optimised in real time as you edit and play.
Check the video below for some random sound examples: