Stackable Gallon Bottle

A simple modification - an indentation at the bottom of a 5-gallon bottle allows it to be stacked.  While consuming more plastic per bottle (due to the increased surface area needed), the space-saving implications should outweigh the added cost.

While only a maximum of two or three jugs can be stacked when filled with water, the stack of empty bottles can go arbitrarily high.

Source: Yujin Kim, via Yanko Design