We can learn a lot from the past – you may even experience aha moments. Reading the source code of the first edition of Unix is such an experience. The simplicity of it all, combined with the knowledge that this actually worked more than 40 years ago.
The github repository (mirrored from google code) contains gems such as a early C compiler (described by Dennis Ritchie here, courtesy of the wayback machine) as well as a beautifully simple implementation of printf.