What the title says.
I’m personally using LMMS (Linux MultiMedia Studio) which despite its name, also runs on Windows.
It is free, it is open-source, and it’s available on the biggest linux distros. It has a lot of capabilities, although depending on tools you’ve used in the past, it might have a learning curve. There are plenty of tutorials online.
Keep in mind this is a tool that one would need to spend time learning, and not one that you can learn in 5 minutes. Depending on your knowledge in audio and music theory, it might get easier/harder to get used to.
Caustic is also a great app to make music upon, though in the Android (and iOS, maybe) versions, you need to have an unlocking key installed to unleash all the remaining features. Moving on to the next, this DAW-in-an-app is good for live gigs, and if you want to experiment with natural and unconventional sounds without the hassle of having to install external plugins, as everything you need in music production, from vocoder to modular synthesizer to supersaws, are all self-contained.