Well, 4-in-1: it's all about the built-in collection of tiny 6502 programming gems. The Pocket Museum.

It's been six years since I made the KIM Uno. For about $10 in parts cost, based on a hidden Arduino Pro Mini, its purpose (purpose?) is best summarised in a picture:

Inspired by my late, great friend Marcel van Kervinck, who hacked Apple-1 compatibility into just 930 logic gates with his Gigatron, this is my rather more modest attempt at Apple-1 fun, but it is pocket-sized. The software does not even need the PCB; all it takes is an Arduino, esp32 or STM 32 Blue Pill over a serial port.

The main benefit of the Apple-1 feature is that you can run WozMon and (thanks to some archaeology of Mike Barry) the Baum/Wozniak 850-byte mini-assembler. You can flip back between KIM-1 hex coding and the interactive mini-assembler with just a keypress. For when you don't know the hex code for that 6502 instruction. 

The other updates: the EEPROM in the microcontroller now acts as solid-state cassette tape for storage.

And the code now runs on the more powerful Blue Pill and esp32 as well: I hit all the limits of the atMega328 (RAM, Flash, speed) and although I much enjoy shaving bytes off code to make things fit, it can't hold any more than it does now. Whilst the future of 6502 development, as is well-known, is a bright area of growth... All details & download on my KIM Uno web pages.