One of the long-running projects involves a sensor which includes a piezo disc: effectively, a specialized microphone. In fact, the output gets recorded as digitized audio, using a computer's or iThing's microphone port.
But, it needs amplifying. The traditional solution is this. Pretty flexible, but on the large side, and it needs its own battery.
Basically the same concept, except that I dusted off some good design practices vaguely remembered from high school (there's a reason for that bypassed resistor in the emitter circuit!), looked up a transistor characterized for low-voltage, low-current operation with moderately high gain (and factory-screened for a reasonably narrow range of gain), did some actual math, spent a few minutes throwing the design together in EAGLE, and sent the board file and three bucks for six copies to OSH Park. Hey, it's 0.35" by 0.875"; not a lot of square-inchage to it.
And it works, first time! As designed, it takes input from a piezo mic, and amplifies it nicely, with the iThing's mic port providing the power and the load resistor.
Might become a product, even. It'll probably lose R1, R2, and C1, though: they're meant for biasing an electret mic, and, as powered by an iThing, there's just not enough power for both this and a typical electret module. The preliminary respin is already down to 0.75" long; if those three components go away on account of pointlessness, it'll shrink even more, maybe to 0.35" by 0.625" (main problem then being that there's not much room on the back for the product name and my name).
Updates: (1) respin, now 0.625" long, is out to fab, and apparently got a spontaneous expedite on account of there was space on an expedited panel; (2) preliminary testing with the client shows that this works, in his application, at least as well as the Spark Fun product, with the possible exception of very low frequencies.