Straton is an image processing software that is used by a large community of astronomy photographers, see sample images.
Jetpix is an image compression library that is able to store still images in much less data than JPG. It was evaluated by German magazine c’t in 1999. Re-evaluations happened 2017 and show that it is still able to outperform libraries like JPEG2000.
Cerebellum & Brainfish
Cerebellum is a chess opening library based on a complex automated generation method. It is updated regularly and proofed to be among the leading libraries. It was chosen to evaluate Google’s AlphaZero learning algorithm, see DeepMind Publication.
Aristarch was developed in 2002 and was the first non-commercial chess engine that was able to defeat Crafty. It is still operational and can be used in current chess GUIs.