A recent high-school grad named Jack Eisenmann homebrewed his own 8-bit computer, wiring up a custom CPU and GPU, writing an OS and hex editor, and producing a suite of demo apps including some fun little games:
Jack Eisenmann, a programmer who just graduated high school, has built his own 8-bit homebrew computer completely from scratch using an old keyboard, a television, and a ton of TTL logic chips. No, he didn't buy some computer parts and snap them together; he blueprinted every wire and connection and then built it, wire by wire. After he finished construction, he had to teach it how to communicate, so he created his own operating system and wrote some games for it. That's dedication.