I'm sure you've read my articles or posts here on this blog re: biofuels, in the form of manure, compost, and others. Today I'd like to mention sewage... yup sewage. Before you turn green and close the window, let me assure you it is possible to get a great deal of energy out of plain old sewage in a safe and sanitary way.
There are many exeprimental programs in place around the world, such as a large biomass power generation project in Japan that focuses on using sludge from sewage plants - this one in particular is so new it doesn't actually open for operation until 2019. However, this plant does expect to generate just about 13 million cubic meters of methane gas annually (enough to power 5,000 homes).
There are other projects out there that use sludge in a slightly different way - they collect hydrogen from the methane gas and that helps in the production energy to fuel cell vehicles, forklifts and other such machines.
The "waste" product is simply processed sludge that can then be sanitized and used as fertilizer etc.