Sorry this is not a complete article, just a few links of interest for those who might be looking to provide power to a Raspberry Pi using alternative methods. We may add to this list later on if we find any more good resources on the subject.

How do I supply power through the GPIO? (Raspberry Pi Stack Exchange) — note that the short answer is, “you shouldn’t”, but the discussion is worth reading.

Troubleshooting power problems (from the R-Pi Troubleshooting page at elinux.org) — more information about how the Raspberry Pi is powered.

RPi 5V PSU construction (elinux.org) — for those that might want to build their own.

Running The Raspberry Pi On Batteries (Dave Akerman) — includes several tips for cutting power consumption.

Raspberry PI UPS (jkliemann.de) – an uninterrupted power supply for a Raspberry Pi.