Boost converter 100mV to 5V
Linear technology makes a few ICs that can boost extremely low voltages up to a usable level for energy harvesting purposes:

http://www.linear.com/product/LTC3108

Alternatively, you could roll your own solution. Such a low suply voltage will require using a JFET or a depletion mode MOSFET as the active element. I designed a self-resonant converter using a depletion mode MOSFET, a few transistors, and an ISDN/ADSL transformer a while back that I could make work down to 0.3 volts and might be able to go lower, but it would need some work.  There are also several patents which are out of protection online like this one:

https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US4734658.pdf

Which you could use as a starting point.