Voltage Controlled Second Order Low Pass Filter
Chapter 6 of Chamberlin's book "Musical Applications of Microprocessors" is a good introduction to the circuitry of classic analogue filter modules (the circuits originating in the Electronotes newsletter). In particular, it has a good explanation of how using circuit elements which act as amplifiers with voltage-controlled gain (such transconductance op-amps) leads to a voltage-controlled cutoff frequency in a VCF. The type of filter discussed by Chamberlin is commonly used for "state variable filter" modules in analogue synths.

The book is out of print, but some kind soul has recently put a PDF online (currently the top google hit for the title of the book).

The Gakken / Monotron filter does avoid transconductance amplifiers, but is not so easy to understand -- the page of Tim Stinchcombe is the best reference I have seen.