If you've ever been interested in Eurorack, this free software is the best place to start.
VCV Rack is a standalone application created by Andrew Belt, a young developer who developed a strong interest in the world of Eurorack modular synthesis. According to interviews, Belt wanted a way to work with modular while on the go and started VCV Rack as a pet project a few years ago. One of the foundational goals of the project was to keep it free of any license restrictions, even forgoing the GPL-licensed JUCE library in favour of developing his own alternative components. Accordingly, you can download VCV Rack completely free of charge from the VCV website. There you'll find versions for Windows, OSX and Linux. Alternatively, you can even clone the entire source code off of GitHub and create your own variation. Someone was even able to make it run on a Raspberry Pi!
Installing VCV Rack is very straightforward, and when you launch the app for the first time you see what one would expect from any modular synthesis application: an empty rack. One thing that may frustrate new users is the fact that VCV doesn't ship with any example presets or patches, but you can find some good example patches on patchstorage.com to get up and running. When you're ready to start building your own patches, start by right-clicking on an empty space in the rack or by pressing enter to bring up the module browser. This pop-up window lets you choose from your list of favourites, or filter by either module manufacturer or search tags. By default, VCV ships with two default packages: Core, which includes the base modules required to send and receive MIDI and audio, and a package called Fundamental, which gives you the basic set of modules required to design synths and effects. You'd be surprised what you can build just using the Fundamental modules alone, and I'd recommend new users do just that before jumping into the world of VCV plug-ins.
Plug-ins are a way for third party developers to create their own modules for the VCV ecosystem. There have been an impressive array of plug-ins created in the short time since the application was released, and the list of what's currently available can be seen on the VCV website. Developers can either list their plug-ins for free or sell them through the VCV site with a premium designation. In most cases, installing plug-ins is a simple process, which starts by creating a login on the VCV site. The VCV application connects to that same login, so that plug-ins added to your account on the VCV site will be automatically installed to the application via "Update plug-ins". I say "most cases" as there are some plug-ins that can't be installed this way. You'll need to download these from the developers' own site or GitHub repository, a process that can be a bit confusing at first. The way the plug-ins are listed on the VCV site can be a bit intimidating to new users as well, as there's no way to sort by popularity or plug-in type to zero in on the packages you really want to add. This problem will only get worse over time as more and more developers get into the game, but I'm guessing this is already on Belt's radar. My list of recommendations for any VCV user would be the Audible Instruments package, which include recreations of the excellent Mutable Instruments line of modules, as well as those from Befaco, dBiz, mscHack, E-Series and Alfredo Santamaria.
From a DAW integration perspective, VCV Rack has more in common with a physical Eurorack modular rig than something like Reaktor Blocks or Softube Modular. This is due to the fact that there is currently no way to run VCV as a plug-in, and from the previously referenced interview, it sounds like this may never be an option. There is, however, a VST/AU effect plug-in available called VCV Bridge that allows you to route audio to and from your rack session. This plug-in is slated to also support MIDI and DAW clock transport in a future release, but for now there are alternative ways of accomplishing this. You can use a loopback utility like the built-in IAC on OSX or LoopMIDI on Windows to send MIDI to VCV. If you're using Ableton Live you can use a VCV plug-in called Link by Stellare Modular, which enables the rock-solid timing of Ableton Link for clock synchronisation.
It's not often that I use the word revolutionary when it comes to music production software, but VCV may live up to that description over time. With a combination of zero-cost price tag, excellent sound with low CPU requirements, stability and the intuitiveness of the Eurorack patching workflow (compared to something like Pure Data), the adoption rate is likely to quickly expand. It's not unimaginable that it'll get picked up by individuals who otherwise wouldn't get the opportunity to explore the world of modular synthesis, and as a result bring experimental concepts to mainstream music circles. The community on Facebook, Reddit and Discord are by and large full of supportive folks who are happy to answer questions, and there is a wealth of good demo videos and tutorials on YouTube already that showcase what's possible using VCV. Simply put, if you own a computer and are interested in modular synthesis, there's really no reason not to give it a try.
Ease of use: 4.0