Ken Stone’s Modular synth site is full of great resources for building synth circuits. One of the many schematics there is a keypad to VC converter which uses a keypad scanner chip to read buttons pressed and convert them to resistor values that attemuate the voltage. I can think of quite a few uses for something like this when it comes to controlling experimental oscillators. This circuit only spans an octave however if more range is needed there are ways to build a more elaborate full function keyboard as detailed on http://www.musicfromouterspace.com
website.