Space elevator, here we come!:

Originally posted by Thequ1ck
I dunno. Gotta be some kind of material that absorbs heat and emits light out there though.

My guess would be Sodium acetate.

'Sodium acetate is also used in consumer heating pads or hand warmers and is also used in hot ice. Sodium acetate trihydrate crystals melt at 54°C,[5] (to 58°C [6]) dissolving in their water of crystallization. When they are he ...[text shortened]... are dissolved; they can be reused many times.'

http://en.wikipedia.org/wiki/Sodium_acetate

That is way cool in itself but there may be something better:

Thermoelectric generators. There have been some fundamental advances in these devices lately, here is one site projecting that technology being a 3/4 billion dollar enterprise in ten years.

http://www.energyharvestingjournal.com/articles/thermoelectric-generators-a-750-million-market-by-2022-00004631.asp?sessionid=1

Originally posted by Thequ1ck
I dunno. Gotta be some kind of material that absorbs heat and emits light out there though.

I am still not sure what you plan to do. Are you saying you would take an asteroid, place a machine on it that converts heat to power we can use and harvest the heat produced as it plummets through the atmosphere?
If so, you are missing a very important point. The heat it produced via friction between the asteroid and the air. A much more efficient way of harvesting this energy is a propeller (which is how we harvest wind energy in general).
So, simply mount a larger propeller on the back of an asteroid and it will generate electricity as the asteroid falls to earth.
But I am not convinced that this energy would be worth the effort. There is no shortage of energy sources on earth, the only problem is harvesting cost relative to coal. As long as coal is cheaper, that's what we will use. This is exacerbated by the fact that we subsidise coal and other fossil fuels.