Energy
The energy available from solar and geothermal alone far exceed our current and likely future energy requirements and could sustain humanity indefinitely. Also steadily increasing energy efficiency 
due to improved system design and increasing cultural awareness should become a significant factor in our energy usage.
The issue currently is commercial economics. The bottom line is that with the current economic framework it is still 'cheaper' to pump oil out of the ground and burn it to produce power than use other more plentiful, renewable and environmentally benign sources. These alternative energy sources are sitting right in front of us waiting to be harnessed. It may be that open-source methods can by-pass the incumbent economic system to enable plentiful, environmentally-friendly power.
We have these major sources of energy available to us, in no particular order and not including fossil fuels that we currently rely on for the majority of our energy today:
Solar:
[1] photovoltaics , solar thermal (such as power tower & [1], ocean thermal energy conversion , SHPEGS and solar updraft tower ). Prospective: Space solar power including solar power satellite , ubiquitous PV and stratospheric solar array
Wind:
land-based wind turbine, off-shore wind turbine. Prospective: jet-stream sky windmills
Ocean:
wave, tidal, ocean currents
Hydro-electric
Nuclear power:
fission Nuclear fission fission http://www.adciv.org/upload/thumb/4/4d/Wikipedia_logo.jpg/11px-Wikipedia_logo.jpg (currently employed). Prospective: nuclear fusion , accelerator-driven thorium-fuelled energy amplifier , and Travelling wave reactor
Geothermal:
shallow geothermal heat pumps , volcanic related geothermal and deep geothermal - Enhanced geothermal systems (EGS). See also Future of Geothermal Power (in the US) published by MIT and Google's funding of enhanced geothermal [2]
Biomass (carbon-neutral):
biofuel (algae), compost methane, fermented crop waste, algae, sustainable wood, and clean burning of: organic waste, animal dung and rubbish
