Maxime Chevalier-Boisvert has created a free/donationware online modular synth called Zupiter that sounds great and is a lot of fun to mess around with. It includes all the basics you’d expect and can be controlled via your computer keyboard, the app’s programmable step sequencers, or externally via MIDI. You can even map your external controller’s knobs to the app’s virtual ones. While the site’s About page say, “Its interface is designed to be minimalistic and simple, so as not to put off beginners”, there is a bit of learning curve. Luckily, users can (and do) post their patches for others to load and play. It’s a great way to get your head around the interface or to hear what it can do. Check it out!