This [voltage booster](http://www.ebay.com/itm/LTC3105-Energy-Harvester-demo-module/332136264440?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D40130%26meid%3De7f86a5b8374439ab3c85d2657cfcb85%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D332071662285) on Ebay takes input as low as 225mv and boosts it to 3.3v or 5vdc.
This can be really useful for boosting power from solar panels on a cloudy day, thermoelectric generation, using the last bit of juice from batteries, and more! WARNING: do not over discharge lithium batteries! They can be permanently damaged. A booster like this would be for like AA alkaline batteries.
The seller, ceech123, has several other similar boards, one even for 2.7v supercaps.