Just for the fun of it, Paulo Constantino dedicated a week to building an 8-bit procesor on breadboard. The concoction looks like one giant cable mess, it's programmed via DIP switches and it can perform a variety of tasks:
All programming is done via DIP switches, and the CPU can perform tasks like counting from 0-255, or playing an off-key-but-recognizable-variant of the Mario theme. No shade for the wrong notes; I can’t even play the Mario theme on a piano, much less program something else to play it for me.
His next project is a 16-bit CPU, which should be capable of running MS-DOS or Linux. His goal is to make the system run light-weight server software, to host retro software, schematics, and a chat room for retro computing fans.