Imation has (or had, it seems to be discontinued) a very cheap (in some markets) storage unit that is stackable called the Disc Stakka. 100 DVDs per unit, powered via the USB cable. I seem to remember it was designed by an Australian company.
I see there's a Dacal Powerdrive which stores 100 discs per unit, and includes a reader/writer in each unit. Stackable to 5 units, extendable to 127 units. Like the old Imation product, it creates a database entry when you stick in a disc it's not seen before.