Went to a car boot sale this morning with the intention of buying something to modify/hack for a Sunday project, was thinking of getting something to circuit bend or fix up. Long story short the boot sale was rubbish, nothing of any interest - unless you wanted to buy grubby kids clothes or broken excercise equipment, the type that you see on late night infomercials 
Walking back to the car I noticed a metronome on the last stall, so thought, well at least I got something that will be useful, paid £2 and went home.
It is a basic mini clockwork metronome and it works ok, quite a pleasing tick sound too, then I though to attach a piezo disc soldered to a 3.5mm jack to it using a rubber band to see if it could be used as a sync generator, and yep it can, quite a simple and easy hack, brief demo here: