Solar Power Satellites
In our modern world we take satellites in Outer Space almost for granted in navigation (GPS), weather monitoring, general communications and simple entertainment. The concept of producing energy in space has been investigated off and on since Peter Glaser invented a method of getting that energy down to Earth via electromagnetic beaming. See this Wikipedia article for technical details.
The first detailed reference study on solar power satellites was performed jointly by NASA and the Department of Energy during the administration of President Jimmy Carter. Using 1970s technology projections, the concept seemed expensive, but achievable. An Office of Technology Assessment review found many uncertainties remained, and estimated that another 20 years would be needed to reach economic feasibility - by which time they expected fusion to be practical.
Some research has continued overseas, particularly in Japan. A restart in the US late in the Clinton administration was making some interesting progress, under NASA's space solar power program. This was reviewed by the National Research Council reasonably positively in 2001, with the caveat that practical deployment depended on greatly reducing the cost of getting to orbit. Unfortunately this program was cut again in late 2001; some of the activities may be continuing under other names with NASA's new exploration emphasis.
A strong and continuing advocate for space solar power funding has been California congressman Dana Rohrabacher. Recent Senate hearings by John McCain's Commerce Committee relating to the proposed return to the Moon included David Criswell, an advocate for a moon-based version of space solar power ("Lunar Solar Power"). Resources from the Moon would likely be a key part of any long-term space power program. The long term future of this part of Space Policy is up in the air.