I started work on Teem [1] as a grad student 25 years ago; a coordinated set of C libraries for scientific visualization. It includes the original implementation of the NRRD file format [2]. One of my goals for this summer is to finally finish a version 2, so I try to spend a little time every day whittling down my todo list for that release. Currently fixing some things in the command-line parsing library ("hest"). Hearing about other people's long-standing projects is encouraging.

[1] https://teem.sourceforge.net/ but these docs are super outdated

[2] https://en.wikipedia.org/wiki/Nrrd