Krita

Krita is a digital painting application built for artists rather than photographers, and that focus is the whole point. Over a hundred brush engines, stabilizers for shaky hands, wrap-around mode for seamless textures, a proper animation timeline with onion skinning, and HDR painting. It reads and writes PSD. GPL-3.0, cross-platform, and the current stable release is 5.3.4.

Installation is a download on Windows, macOS, and Linux, with an Android tablet build as well. Nothing to configure and nothing to run. It expects a drawing tablet: pressure sensitivity is where the brush engines earn their reputation, and using it with a mouse wastes most of what makes it good. GitHub is a read-only mirror: bugs and patches go to KDE's Invent instance, and pull requests opened here get closed unmerged.

Illustrators, concept artists, and animators should use this instead of Photoshop or Clip Studio Paint. It is not a photo editor, so retouching and compositing belong in GNOME/gimp. Studios running Windows can put it on every machine at no cost and no seat counting.

The catch is where the money goes, and it is worth understanding. Krita is free from krita.org, but paid builds also sit on Steam, the Microsoft Store, Epic, and the Mac App Store. Those are the same GPL program; you are buying a donation with automatic updates attached. Paying is a choice to fund development, never a way to unlock anything.