Things to build from scratch

HTTP server

Shell (in C)

Prompt loop, input parsing, fork()/execvp(), cd builtin, pipes. Done when it can navigate the machine and run normal programs

Compiler

Maybe an interpreter or a build system too

Kernel

Distributed system

Design, build and deploy one

Emulator

Self-replicating virus

Ray tracer

Spheres, a light source, shadows; output a .ppm file