YAAMIDRUM (Yet Another Midi Drum) is an open hard- and software project based on Arduino (http://www.arduino.cc/). Supports 48 inputs for piezos or other sensors and optical hihat with infrared sensor. Cheap and easy to build.
I'm not able understand how did author of this project got rid of ringing issue when drum hit occurs. I have a exponentialy decaying ringing signal is observed on similar setup with piezo transducer. In my project every drum kick used to trigger atleast three MIDI midi messages.