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 became part of Perkin Elmer, and recently it appears the Vactrol trademark has passed to a company called 'Excelitas'.

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) (Note: current spec sheets are to be found at the Excelitas page linked below.)

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

Manufacturers
Vactrol product page at Excelitas: Analog Optoisolators and Optocouplers

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

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

There are reports of success using the Fairchild H11FxM Solid State Relay Optocoupler family. These devices use a silicon photo-detector (rather than an LDR), which behaves like an FET, and so they don't suffer from the lag in response that LDR-based devices have, i.e. they can be modulated at audio rates!