Krita is a paint and drawing tool, first conceived for Linux back in 1999, but now also available for Windows and Mac.
In its early days the program was mostly about image editing, and that's still obvious today. Even if you've no interest in painting, you can just open an image, resize or crop it, apply effects, add annotations and more.
The real power here is in the painting tools, though. The hugely customisable brushes, the colour mixing, tablet support (Wacom, Huion, Yiynova), helpful drawing assistants, layer support, blending modes, versatile selection tools and more.
The package is now a capable animator, too, with multiple layer support, onion skinning, playback speeds and more.
Version 3.1 release notes.
A versatile and comprehensive painting tool for experienced digital artists.