This guy, Ben Eater, is a pro at teaching to use your brain, and has videos of some unique things.
His "Hello World from scratch" 7-video series is now complete, but you can also browse his site
, where you can follow him while he creates a Frankenstein-type processor/computer with just plain old TTL chips spanning several breadboards. It is a Frankenstein, in my humble opinion, because it mixes up the processor with RAM (can you believe it is a Turing-complete processor/machine with 15 bytes of RAM?)
I guess these particular two video series (the computer-on breadboard spans forty-something videos, you've been warned) can help those that sometimes are afraid of "not knowing enough" by giving'em the slight push someone needs to embark in some project/repair. And Ben explains things really well.
Not only the videos: If you feel like it, he sells kits for both tutorials (6502 and computer-out-of-TTL).
Another example, not related to Mr. Eater, is The Gigatron, another discrete-TTL computer without processor. But I feel it kind of cheats because for some things it uses a microcontroller in some port (was it a serial port?).
And for the really extreme... Just take a look at this series
: Nothing less than almost 30-video series of people restoring an old, trashed Apollo Guidance Computer (AGC). We're talking not only of a processor-less computer, but one made exclusively of three input NOR gates, two gates per 10-pin flatpack chip that must be reliable enough to guide a rocket! This means no shift registers, no latches, no counters, no "complex logic", as every building block is made up of NOR gates.
By the way, AGC's ROM and RAM was made up of thousands of really small ferrite rings with copper wires interwoven; and RAM was volatile: The machine had to WRITE back a value after READING it, otherwise it would be lost. Not dynamic RAM, but "suicidal" RAM.
Ok, I better shut up; my age starts to show