The ISD1000A is a specialized microcircuit in a 28-pin DIP design. It is almost a complete voice-recording system. Add in a microphone, speaker, a couple of resistors and capacitors and you have a discreet recording and playback device. It handles 20 seconds of audio, and makes a great little practical joke.

You can pick them up at places like Digi-Key or Radio Shack for about $15.