In Oslo, they've started using raw sewage to heat their homes. Heat is extracted from the sewage and transferred to hot water pipes - which then send hot water back to homes around the city. The technology is basically the same thing that makes a fridge work, and the city is suddenly able to heat the equivalent of about 9000 apartments per year - and save about 6000 tonnes of oil that would have otherwise been burned..
It sounds as though the technology could be used all over the place, which is way cool. So far the only problem in Oslo seems to be the irregular water flows. As in, there's not much raw sewage available early on Monday mornings (4-6 AM) because most people go to bed early on Sunday night.
Tangentially, my favorite quote from the whole article is this:
"In Oslo, untreated sewer flows -- from toilets, bathtubs, sinks and rainwater from the streets -- runs into the system past a filter that keeps out big objects such as dead rats."
There's just something grisly about the "big objects such as dead rats" bit.