How to Generate Power From Your Daily Activities

Two recent posts speak to the potential of a rising trend: converting small amounts of human energy into usable electricity. The first is a proposal from a design firm in New York to turn the energy expended rotating a revolving door into power. The second is borderline ridiculous, but well-intentioned: an "energy tail" you strap to yourself, which generates power from the friction of two rollers against the ground (to mitigate some of the absurdity of this, it was designed for rural Africa, where energy is in short supply).

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I wonder what the potential impact of technology like this really is. More specifically, is the cost to develop and implement this technology worth the potential energy savings it engenders? Follow up question: Did someone at Toyota have the same attitude when an engineer suggested capturing the energy produced while braking to charge the Prius's battery? I'd be curious to know if these incremental energy saving could really add up to something big, or if instead this is a technology designed to create a new marketplace, but not real solutions.