A currency backed by energy. This type of currency could be used to create an energy economy - a method for a local currency to be implemented in any community. The problem with many local currencies such as LETS  or Ithaca hours is that in businesses that are not a part of the program the currency is useless. However, if the currency can be used by ANY business or community member it can scale. A currency backed by renewable energy achieves this goal. The technology is finally available to make this happen.
Sources for Further Reading on the general topic
- SolarCoin 1MWh of solar electricity based on bitcoin technology
- DeKo Transitional Energy portfolio proposal for central banks SSRN Paper
- Local Energy - Local Currency - Local Power by Rob Hopkins
- Shann Turnbull in articles called Selecting a Community Currency
- Democratising the Wealth of Nations - from new money sources and profit motives
- David Boyle’s “The Money Changers”
- Energy as a currency at Energy and Life
- in the Schumacher Society by Robert Swann, called “The Place of a Local Currency in a World Economy”
- World Kilowatt Dollar (note that a kilowatt is a unit of power not energy.)
- The Perfect Currency a site dedicated to energy currency
There has also been significant discussion in the literature about an "energy currency" based in the hydrogen economy but this is more focused around an economy run on hydrogen rather than say oil not as the the hydrogen being the means of exchange.
- Local Currencies at Wikipedia
- An International Energy Clearing Union 
- Energy Theory of Value Literature Review