A collection of small or minimal CPU designs and computers by Jack Eisenmann (found by Zaphod) - often with support such as emulators, C compilers, etc:
DUO Compact
A single-instruction CPU, 8-bit, TTL, fully designed and constructed with operating system and test applications.
DUO Basic
An educational homebrew computer, prototype has been completed.
http://www.ostracodfiles.com/basic/IMG_1391_small.JPG
DUO Light
An ATMega328 based computer, prototype has been completed.
DUO Decimal
An ATTiny84 based computer (has been for sale but now unavailable)
https://www.youtube.com/watch?v=G5EbSjDjMG4
DUO Portable
An ATMega1284p based computer with LCD and qwerty keyboard
https://www.youtube.com/watch?v=MSd0tkPFdMQ
DUO Capitol
8 bit microprocessor design. Not built but has an in-browser emulator
DUO Adept
Ambitious 8-bit TTL machine built in a storage box
DUO 128 Elite
4-bit computer, built from 40 chips, with 64 nybbles of RAM, toggles + LEDs on front panel
DUO 14 Premium
Relay machine, 55 relays, 48 bits of ROM and 14 bits of RAM
