Look up Joule Thief some time. There's a MAKE Article and Youtube about making a bracelet that uses sunlight on photodiodes to charge 3 caps that a Joule Thief made mostly from a small choke (you don't even have to wind coils) to light a led you can see in daylight.The full article: http://makezine.com/projects/Solar-Joule-Bracelet/The YT: https://www.youtube.com/watch?v=ghB2irHIN8I
I'd also recommend pololu.
it seems they're not on the open source bandwagon.
Maximum quiescent current: 1 mA*The highest quiescent currents occur at very low input voltages; for most of the input voltage range, the quiescent current is well below 1 mA
Some of Pololu's boost regulators work down to 0.5 V. Pretty hard to beat that, especially considering the low price.http://www.pololu.com/category/132/step-up-voltage-regulators
While researching different types of regulators for a battery powered application, I learned about jeelabs and how they use a step-up voltage regulator to suck every pick of juice from AA batteries.
The problem with that approach is that it trashes rechargeables to discharge themso deeply and I never use anything else.