Belgian economist Bernard Lietaer is one of the leading researchers on complementary currencies. Here an interview with Lietaer:

Here a speech by Lietaer on money diversity:

The main point made by Lietaer that a society that relies on one single [type of] currency is unstable and by having multiple types of currency we would have a more stable economy. Different types of currency would serve different sectors of society.

Here a link to Lietear’s website:

Rethinking Money