The Radio Shack Mini-Amplifier and Speaker was a staple of the Exploratorium Teacher Institute, used in a number of activities (which we call, "Snacks").
;)
It was an inexpensive, fairly loud, if somewhat garbled external speaker. Its main feature is that the internal amplifier will amplify the tiniest of signals that most other amplifiers ignore. It made the beige wonder a bit squacky if a connection was poor. You can find it used in older versions of Modulated LED where a solar panel is connected to its auxillary input. With the demise of Radio Shack, the Mini-Amplifier and Speaker has also gone away.
We first tried computer speakers as a replacement, but they don't work well. In order to get more clarity and less hum, they cut out low amplitude signals. The signals that we are picking up from solar panels are teensy, so in these won't work.
There is a replacement, however, which works well. You want a voice amplifier, a device that is often used by people who need to have more voice but not a full PA. You can find them on Amazon for between $20 and $30. You can often find them for less, sometimes, at your local DJ supply outlet. I got the one below for $15 at an electronics store in a downtown Los Angeles.
;)
The key feature that you are looking for is a microphone input. Obviously things that come with microphones will have one, and see the part that says "MIC" below? That's the ticket. Microphones have tiny voltages and so the internal amplifiers need to be extra sensitive.
You can really see the difference in this model. Plug the solar panel into the AUX-IN, and the speaker produces no sound, while plugging into the MIC works perfectly.
;)