Electrosmash brings an opensource Arduino based guitar pedal. Effects can be changed by downloading precoded effects. They have made some sample effects already.
pedalSHIELD is a programmable Arduino Open Source guitar pedal made for guitarists, hackers and programmers. Users can program their own effects in C/C++ or download ready effects from the online library.
It is designed to be a platform to learn about digital signal processing, effects, synthesizers and experiment without deep knowledge in electronics or programming.