Besides the already mentioned reverse-polarized diodes as varicaps
(http://www.hanssummers.com/radio/varicap/index.htm), I use 74HC240 as HF
QRP finals (600mW) and IRF510 as HF PA (20-30W max, QST has some articles
in the past).

Paolo IK1ZYW

TL-431 Shunt regulator as a low powered fixed gain 3 terminal audio
amplifier.

W4ZCB