Free screenshot software

ImageMagick, invoked from the command line as magick, is a free and open-source cross-platform software suite for displaying, creating, converting, modifying, and editing raster images. It can read and write over 200 image file formats and is widely used in open-source applications. ImageMagick was created by John Cristy in 1987.

ShareX is a free and open-source screenshot and screencast software for Windows. It is published under the GNU General Public License. The project's source code is hosted on GitHub. It is also available on the Microsoft Store and Steam.

utility used in the GNOME desktop environment for taking screenshots

Greenshot is a free and open-source screenshot program for Microsoft Windows. It is developed by Thomas Braun, Jens Klingen and Robin Krom and is published under GNU General Public License, hosted by GitHub. Greenshot is also available for macOS, but as proprietary software through the App Store.

In the X Window System, the program xwd (X Window dump) captures the content of a screen or of a window and optionally saves it into a file.