Many of my consulting projects revolve around writing custom software to run custom manufacturing and test automation equipment...
I have shared a couple of C libraries I have written for the Atmel AVR 8-bit RISC microcontroller family. One provides functions for USART communications, SPI communications, "gentle" EEPROM updates, and network "endian" conversion. The other provides a preemptive multitasking/multi-threading library written mostly in C.