1. A stacked integrated circuit comprising: a first circuit layer; and a second circuit layer overlying the first circuit layer, wherein at least one of the first and second circuit layers comprises at least one integrated circuit; and at least one interconnection between the first circuit layer and the second circuit layer, passing vertically through the at least one integrated circuit; wherein at least one of the first and second circuit layers is selected from a stock of a plurality of different interoperable, previously-fabricated circuit layers.