Tapdrum Example
Tapddrum Basickit2

The Tapdrum kit coverts 8 analog inputs to assignable MIDI notes via an Atmel 89S8252. The inputs are intended to be used with piezo sensors (optional with kit) but could foreseeably use a number of different sensor input types, even audio as Deviantsynth suggests – Tapdrum kit

The site also has a quick how-to on making your own piezo sensors – BUILD PIEZO TRIGGERS(DIY)

Related:
Beatbox2 800
The $5 drum machine