This Desktop Energy Seed Lamp is interesting. It uses old batteries as a power source. All of the batteries are put in parallel using two large metal plates and a jewel thief circuit (I prefer the rusty nail implementation though) is used to create the larger voltage needed to power the LED circuit. As long as the batteries all have about the same amount of charge left the circuit should efficiently drain the remaining energy out of the batteries. I am not a big fan of Parallel LED circuits, but they may last long enough not to make that a concern. Have a look at the full article, it documents the mechanical and electrical process of this design very well.
"Today I will show you something very interesting. It is not a killing robot or skynet (not yet). It is a desktop ambiant light that use dead alkaline battery to power itself. This design can hold up to 15 batteries. It use a single joules thief circuit to power 50 LED!"
