[Juan] sent us his writeup of a microcomputer he built using an Arduino UNO (AVR ATmega328p) and some off-board SRAM. This one’s truly minimalistic. Have a look at the schematics (PDF).

They call their creation the AVR General Purpose Programmer, or AVRgpp for short. It runs on an ATmega328P with a Pro Mini bootloader, which means that the programmer itself is fully compatible ...