I've been using Rack for a year now and I can certify that is not a wasted download. Rack is the culmination of expertise in modular synthesis knowledge compiled into the best modular software. The total amount of modules is above 1,300 if you only count the free ones. Of course there are also commercial modules available for very reasonable prices. The sound quality is excellent, and so is the versatility of the signal routing that is known to set true modular synthesis apart from the rest. I highly recommend Rack.
This virtual modular synth is amazing... there are seemingly infinite possibilities. And better yet... IT'S free!!! I would rate this 5 stars, however it crashes pretty much every other time I use it, and when using big modules I experience lag. It is possible to ignore the lagging, however the crashes can become annoying. Despite this, I am soooo thankful to the creator of this software for making it. It is one of my favourite audio applications.
Although I don't own any Eurorack hardware I've seen videos of this thing being compared to it and I can't tell the difference. The latest version will connect to your DAW (though not quite a full-fledged plugin). I think this is pretty amazing software, AND it's free and open-source.