I found an old Canon Canola L100 calculator at Recycle North that I thought would make a great instrument. It lay around in my attic for a couple years until I needed a housing for a square wave generator I was working on.
The square wave generator is about as simple as it gets - I found an assembly program at http://chip.aeug.org/other.html to output a 12-tone octave. Program it into a PIC 16F54 and it will toggle 12 of the pins in full scale of semitones. I have a PICKit 2, which is a pretty cheap way to program PIC processors.
I needed more than 12 notes, so I made two versions of the program and programmed two different PICs. I put eight notes on each PIC, and left four empty pins on each in case I use them for inputs in the future.
I wanted full polyphony, so I put resistors on all the pins and ran each to a switch. The keyboard on the Canola is all reed switches, which close when the key pushes a magnet down next to them. Every one that is pressed mixes its note onto the main output, which goes through the volume control before returning to the main board.
I drilled out some plastic to put where the 110V plug used to go, and mounted the 1/4 inch output in it. I took the top half of the Canola and used the same Krylon Fusion spray paint as on the Eye of the Tiger.
It works well - the sound gets quieter the more notes you play at once, and it's a pain to replace the battery, but it has a nice sharp square wave tone and the keys are very playable.