Krita is fantastic. There's a lot of open source painting and imaging software out there, but Krita really stands out with a cool brush engine, good performance, and a feature set very useful for digital art. I only hope Krita can receive more substantial funding in the future.

Krita definitely helped me leave some closed source apps that I used to use for hobby, without feeling like I was making a compromise (quite the opposite really.)

Of course, it's not really equivalent to Photoshop or Clip Studio Paint, for example, but I think it's competitive in some areas already.