The Herbie isn’t strictly a BEAM bot, but it follows the BEAM tradition of using ICs for purposes they were never intended for. The Herbie circuit is based on the LM386 integrated circuit, which is normally used as an audio amplifier. In the Herbie circuit, the chip is used to amplify the difference between the outputs of a couple of photodiodes, giving the bot phototactic behavior.
Here’s my Solar Herbie. It’s the standard Herbie circuit, with a Miller solar engine. The LM386 is tucked above the blue trim pot (I flattened the pins to make it sit flat). The small motors are from laptop CD-ROM drives, and the solar engine is positioned between the motors and the capacitor.