Big D

Big D is a Mosfet Joule Thief with a CDS photo resistor for turning on at night.

 Watson's eblog and Quantsuff came up with the idea of using a low turn on voltage mosfet.  I found a Supertex TN0702N3-G device that fits the bill at Mouser.

The device has a low turn on resistance for delivering lots of current to the led.  This one is a blue-green led from Electronic Goldmine.

Magnets are used to attach the Joule Thief to the battery. The battery lays on little stick-on bumps attached on a base. You can use AA, C or D size 1.5v cell to power the Joule Thief.

The core is a Fair-Rite 2643002402. You can get them from Mouser for 13 cents.
The circuit is wired up on a small piece of perf board.
Six turns of two wires passes through the core.  Use wire from ethernet cable.

During the day it rests with very low battery drain

PDF of schematic

At night it lights up like crazy