For me, it was perhaps simple-scan, a very simple and efficient GUI to scan documents. I used it with my Brother printer / scanner and it works like a charm. Especially since I do not scan stuff often, so a program with more complex UI would have the effect that I forget how to use it until the next time.
There is a free VST (or clap, etc) synth called TAL Noise Maker (I think) that has Linux support. Good for getting started. Then there is the LSP plugging suite, which is designed for Linux. Lastly, airwindows is native to Linux as well, and is literally one of the best — and 100% free! — plugin suites ever. All of there can be moved (just copy paste) in ~/.vst, ~/.vst3, or ~/.clap.
Links (FYI, clap is always easier than VST, so prefer it; you may have to enable it in settings or something though):
You can of course also use external stuff like Pure Data and Orca through MIDI, OSC, UDP, or even loopback.