A booster based around the Russian GT308 germanium transistor, more info
here. Supposed to be for Bass guitar but sounds good on guitar also. Cuts bass frequencies, and boosts some high frequencies. Some frequency info from the linked thread above -
10 Hz: +2dB, 63 Hz: 0 dB, 100 Hz: -1,6 dB, 200 Hz: -8,5 dB, 350 Hz: -25 dB (notch), 500 Hz: -14 dB, 1000 Hz: -6 dB, 1500 Hz: -3,5 dB, 3000 Hz: -1,5 dB.