TeX-in-C from http://www.math.uni.wroc.pl/~hebisch/tex/

This one still seems to be active (latest timestamp is December 2017), so it may not be at risk of getting lost (does not need archiving, and we should probably check with the author).

Saving it anyway for now. The author has some very pertinent thoughts on their pages: 1, 2, 3, 4, 5, 6