I'm working on a cartridge for the Commodore VIC-20 microcomputer. The cartridge contains a 256KB flash chip which can be programmed from a host PC while the VIC-20 is running. This allows fast prototyping and programming the VIC in a way very familiar to Arduino hackers.
I just finished building a prototype on the breadboard. Next I'm going to design and etch a two-sided pcb for it.
Full write up with schematics, video and more photos in my blog: