Linux

Ignition from Inductive Automation. Works great on Linux, used to run it in docker even. There are drivers for all kinds of PLCs. It's a dream to develop in, was my SCADA platform of choice (I've moved on from the industry). If you need to script anything, it is in Python, not some bullshit proprietary scripting language, nor VBA garbage. The client software is great, even runs nice on PC-based HMI touch panels, which you could install Linux on if you want. The call-out alarming actually works (FUCK WIN911).

The software is free to try and download. You can develop in it for free, unlike the majority of competitors. Go ahead and try all of it out right now if you want. The training courses on using their software are free, nice handy videos, so you can start learning how to build everything like right now.

The "catch" is it costs money to run all of the SCADA critical components for more than an hour at a time (to prevent you from just using it to run your whole plant for free). But you can build your whole SCADA app today with your PLC gear on hand, and only pay for it once you are ready to deploy to production.

Anyway, to me, it's hands-down the best SCADA platform, and it even runs on Linux. Disclaimer: some of this might be out of date, I've been gone about 4 years.

Edit: sorry, didn't see the "free" requirement. I would never run a critical plant without support, so I've not explored any fully open source options. If your plant is serving more than just your farm/homestead (in other words, is serving the general public) I strongly recommend a supported option for your client. If you get hit by a bus, and the plant is in trouble, they'll have a hard time finding someone to get them back online who knows your "weird" software.

That said, depending on your needs, Ignition can be cheap AF (comparatively) if your plant is small.