Lingua franca of programming world. I write Go which is similar to it.
Principles for C programming
c4 - C in four functions.
Project Based Tutorials in C
One year of C
Building C programs
Tiny Garbage Collector - Tiny Garbage Collector for C.
Awesome C - Curated list of awesome C frameworks, libraries, resources and other shiny things.
9cc small C compiler
picol, a Tcl interpreter in 550 lines of C code (2007)
Boehm-Demers-Weiser conservative C/C++ Garbage Collector (libgc, bdwgc, boehm-gc)