Adobe Photoshop Alternatives

Many of the following programs can be used as an alternative to Adobe Photoshop (a popular raster graphics editor).

FireAlpaca, Windows and Mac OS X, free
https://firealpaca.com/en

GIMP, free, Windows, Mac OS X, and Linux
https://www.gimp.org/
This program is very powerful, but can be difficult to use.

Pixelmator, Mac OS X 10.9+ (App Store), $29.99
https://itunes.apple.com/app/id407963104

Pixlr Editor, web-based
https://pixlr.com/
This also has iOS and Android versions.

Affinity Photo, Windows and Mac OS X (10.7+), $50
https://affinity.serif.com/en-us/photo/

Artweaver Plus, Windows, $40 (a limited free version is also available)
https://www.artweaver.de/en/

Krita, Windows and Linux, free
https://krita.org/

Pinta, Windows, Mac OS X, and Linux, free
https://pinta-project.com/