Energy

The energy available from solar and geothermal alone far exceed our current and likely future energy requirements and could sustain humanity indefinitely. The amount of energy that falls on the earth from the sun in a minute is what humans currently use in a year across all nations and industries.

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 , maglev

Ocean

wave, tidal, ocean currents

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

Bacteria

Certain species of bacteria (such as geobacter) deposit electrons onto electrodes placed in their environment. Much work is still being done on optimizing the systems, but microbial fuel cells already provide a cheap and very resilient form of energy. A $40 system developed by Dr. Peter Girguis and Dr. Helen White has shown itself capable of producing 96W of power[3]. This system used inexpensive charcoal electrodes and can run for years and years without maintenance. Since then, a new strain of geobacter bacteria has been developed that has a power output eight times greater than previously known strains[4].

In a post-scarcity world, microbial fuel cells could be synergized  with composting toilets to create a system that disposes of human waste, fertilizes plants for food and also generates electricity.