GNU Image Manipulation Program
GNU Image Manipulation Program (short: GIMP) is one of the longest-running and best-supported free open graphics apps. It offers Photoshop-style raster graphics and photo editing for Windows, Mac and Linux.
Hint: With Glimpse, there also exists a variant with a less mistakable name.
Glimpse Image Editor
Glimpse is a libre graphics application based on the popular GNU Image Manipulation Program. The project tries to improve the performance and user experience of the latter.