I come from a java background (from my CS classes) and a semester of C++. I am just finishing up a OpenCV project for my Co-Op that\'s in pure C, so I\'m a bit late in aski
systemd
systemd code quality tools
RIOT-OS
nginx
Git Source Guidelines
FFmpeg
curl
The Architecture of Open Source Applications C programs there include Git, Nginx, Open MPI, GPSD (C & Python), GDB, FreeRTOS, Berkeley DB, Bash, Asterisk,
VLC (lots of architecture info)
Any of the above projects are worth a read. Standard command-line tools are also available to read. The Linux kernel has a lot of design documents out there too.