Vactrol

In its simplest form the analog optoisolator consists of an LED and a light-dependent resistor (LDR) in the same package: light output from the LED varies as the current passing through it; the light strikes the LDR, varying its resistance in turn; in effect this gives a current-controlled resistor.

An early manufacturer of these devices was Vactec Inc, who used the trademark Vactrol, and this has since largely become genericized to cover all such available devices, whoever might make them (through acquisitions etc., Vactec have become part of Perkin Elmer, who apparently now own the Vactrol trademark).

This Perkin Elmer document contains a good introduction to such devices: Analog Optical Isolators: Vactrols

A more complete Perkin Elmer 'databook' contains not only the spec sheets of the various types of Vactrols, but also has many useful 'application note'-type circuits of how they might be used: Photoconductive Cells and Analog Optoisolators (Vactrols)

There is also much useful info in the Wikipedia article: Resistive opto-isolators

Manufacturers
It is not clear if Perkin Elmer still actually make them - navigating their site is a nightmare: Perkin Elmer

Some Doepfer modules are known to use a Macron devices: Macron specification page

The Silonex 'NSL' series are also attracting attention: Silonex Audiohm Optocouplers