Photopea - A web application that can open Clip Studio Paint files without needing the app installed. (I use it on my aging laptop because I'm worried that if I burn a license key, I'll have to re-purchase Clip, which will suck)

Piskel - I love Piskel because its interface is just about as simple as MS Paint and it does what it needs to do simply, for free, and better than other programs with less approachable interfaces.