I love using piezo contact mics - they're small and cheap, and can do a whole bunch of different things.
Unfortunately, they can sound pretty rubbish. In particular, they don't like long cables - all of the bass disappears, and they get quite "muddy".
With a lot of help from Matt Venn, I've redesigned the board layout with a view to putting it inside bottle-cap hydrophones: as small as possible, and approaching circular.
The current version is always on GitHub: https://github.com/mo-seph/PiezoPreamp (Creative Commons, as per the originally shared versions)
I've had a batch of 100 boards made - if you want to buy one, email me. They'll be unpopulated (i.e. you have to solder components on it), but cheap (£1-2, probably). If there's enough interest, I'll put together some kits with components in. A Farnell parts list is attached.
Matt put this picture of the next round of Piezo Preamp boards up on his Flickr Stream. I've removed the piezo -ve input, and tied it to ground, which might loose 3dB of gain, but makes figuring out ground planes a lot easier. I couldn't hear the difference when I shorted piezo -ve to ground, and it makes the layout much smaller and easier.