Until recently, practically everything "free" was really just the result of what economists would call a cross-subsidy: You'd get one thing free if you bought another, or you'd get a product free only if you paid for a service.

Over the past decade, however, a different sort of free has emerged. The new model is based not on cross-subsidies - the shifting of costs from one product to another - but on the fact that the cost of products themselves is falling fast.

Once a marketing gimmick, free has emerged as a full-fledged economy.