Designs of voltage/current controlled resistance

Hi Friends!
What approaches exist of creating voltage or current-controlled
resistance?
There is no some specific schematic in which I want it to be plugged in,
but I'm designing a few gadgets related to electronic musical
instruments - and sometimes I want such resistance for volume control
while at other times to be used in certain frequency generator to
control the frequency itself.
I vaguely remember there is JFET but care is needed to fit signal into
its small-voltage "ohmic" region and provide suitable bias to the gate.
Also at least for some schematics opto-couple with resistor in the
output part seems viable (though I'm not sure about linearity - on the
other hands linearity is not often needed).
Next there are motor-driven potentiometers but in my context it looks
ridiculous.
Also I vaguely remember tube triode has curious voltage-current
characteristic which probably means it could be utilized in a way
similar to JFET (though it is a bit exotic).
And then there are complex and composite devices, digital resistors etc,
but I prefer to work out some more "plain" solutions as they are in part
for educational purposes (e.g. to be constructed by school pupils).
May I be missing some fine schematic with a handful BJT or something
like this?
thank you in advance,
sincerely yours,
Rodion
--
to email me substitute github with gmail please