VCV rack is awesome, if not the easiest to start from scratch with. For those who haven't heard of it, it's basically a virtual modular analog synthesizer. You can build your own sound generating machines out of a huge selection of modules, including oscillators, filters, sequencers, etc. Some of the modules are based on real hardware modules that you can buy, but there's a huge variety.

It's fun to play around with, but if you're new to modular synthesis then I recommend limiting your module selection to just the built-in ones to start because the full selection is pretty overwhelming if you don't know a VFO from an ADSR evelope generator.