I don't usually link to "giant jumble of links" sites, but there's so much useful stuff here, I think it's worth it. Lie Detector, Theremin, Sensors (airflow, strain gauge, etc etc...), amplifiers, pre-amps, alarms, radio transmitters, power supplies. This would be a pretty good resource if you were making something were you didn't want to design circuits to drive some small part of the project and just use someone else's circuit.

As usual with this kind of site, some of the links are dead, but at least you know it existed on the internet at one point, and then search for something similar...