GIMP

GIMP is a raster image editor built around layers. It covers what most people expect from a full editor: layer masks and blending modes, selection tools, curves and levels, healing and clone brushes, transform tools and a large set of filters. The native XCF format keeps a document editable between sessions, and export supports the usual web and print formats.

The window layout is up to you. GIMP can run as one window or as floating panels, and every dockable dialog can be moved or hidden. Script-Fu and Python plug-ins turn repetitive work into something reproducible, which matters when the same crop and colour correction has to be applied to a few hundred photographs.

The project has been developed in the open since 1995 and ships for Windows, macOS and Linux. It is a sensible choice when the job only needs an image editor rather than a whole subscription suite.