I'd like to present my summer hacking project...
The KIM Uno is an Arduino (Pro Mini) based clone of the vintage 1976 KIM-1 computer. Open-source hardware.
Aimed at vintage computer fans, but an extension to the original KIM also makes it a very useable programmable calculator. Programmed in vintage 6502 machine code. Battery-powered, and it fits in a shirt pocket 
Project goals:
The main goal was to create a faithful KIM-1 experience at minimal cost and effort. I still think you can learn a lot from coding bare-boned 6502s. Parts cost including PCB is about $10: 1 Arduino Pro Mini, 11 resistors, 24 keys and 2 segment LED blocks.
The second goal had to do with the question "what can something as obsolete as a 1976 KIM still usefully do?". That was why I extended the Uno with programmable calculator funtionality. Very short & simple 6502 code can do useful stuff this way, so it should get people coding the 6502 for a purpose...
I also enjoyed a bit of Software Archaeology, by adding ROMs containing things like Microchess, the first microcomputer chess game (coded in 924 bytes!), and the first 6502 disassembler from Wozniak and Baum (505 bytes!).
Site:
The project including schematics, Gerbers and source code is here:
And here's the subpage on the 6502 programmable calculator part I'm particularly proud of :
Some pictures:
During the next 2 months, anyone interested can get the PCB plus parts from me at cost price if they want. Just to make sure: this is done as a strictly non-profit hobby project, so I ask for the $10 that the parts cost me. If interested, leave me a message on my site (link) please.
Oh - even without the PCB, the software should run on any Arduino (tested on an Uno) but without the PCB you can only talk to it over the serial port of course.
Kind regards,
Oscar