These things have recently gotten a lot of attention since someone found out that The Edge from U2 uses one as part of his sound. Originals routinely sell for over $500 on eBay.
The Boss FA-1 FET Amplifier was released in the early 80’s and discontinued in 1985. The original was not a traditional pedal, but instead an always-on effect with a clip on the back, with the idea that you actually wore it and your guitar plugged straight into it before going to the rest of your chain.
My version adds a footswitch and LED to turn it into an actual effect that’s more at home on the floor than clipped to your belt. Otherwise, it is completely faithful to the original circuit. If you’d like to experiment, it also has additional pads allowing you to use a DIP8 dual opamp instead of two stock SIL HA1457Ws as in the original.
If you want to read more about this pedal, Music Gear Reviews has a fantastic article.