+1 on VCV Rack, I use it more than anything else, and my whole first album was made exclusively using free modules there. I do get some of the premium modules there, mostly to support developers who put their time and effort into making the vast majority of the modules free.
I’m currently trying to get into Bespoke Synth, the DAW that describes itself as “If you took a hammer to Ableton and told someone to put it all back together on their own for each project”. It’s got similar UI to things like Max with elements you wire up like a modular synthesizer, but with live-coding in python and VST hosting and such. And it’s free.
I also really want to get into playing around with Vital Synth, I’ve heard it’s really powerful and also free, and can run as a VST.
Finally, I use Minifreak V a lot, as I have the hardware Minifreak and it works well with syncing the hardware with the software, and it also just got the 3.0 firmware update today with sample engine and seven granular algs. Really, really powerful soft synth, but also money.
Befaco is incredible, one of my favorites. They also just released Oneiroi on VCV as a $35 premium plug-in, it’s on my wishlist. I’ve been wanting to play around with it since it dropped in hardware, and soon I’ll be able to. It’s got some differences from the hardware, but from what I’ve seen it seems to be very true to the hardware while using the fact VCV is software and using that to make it more user-friendly. For instance, being able to load .wav files directly into the looper buffer.