Power Line Broadband
Power lines are everywhere, connecting every house—and every computer. Why not project broadband over them? This is known as BPL, Broadband over Power Lines. Think: broadband access everywhere that there's power. Just plugging in your computer to a power outlet would also give it internet access!
Power line broadband has a few problems. The biggest one is that power lines are an inherently noisy environment, partly because they weren't designed to transmit anything other than power in the first place, and partly because every device that is connected to the power line creates its own frequencies and harmonics that can interfere with any data transmission. There are techniques to work around this, however.
Another problem is with the frequencies that BPL utilizes. Because powerlines are unshielded, meaning that anything that gets transmitted through the lines also is dissipated into thin air. The frequencies that are favored for BPL, precisely because they are less sensitive to the usual power line noise, are the same ones that are used for radio, including some international military frequencies. Shielded power lines are the obvious solution, though it would require an expensive overhaul of the current power grid.
Yet another problem with the currently available power line technology is the existence of transformers. BPL can't transmit information directly through transformers, requiring a repeater to be built in to complete the connection. US power grids tend to have a single transformer for every house, making it very cost inefficient, though European power grids tend to share transformers with tens or hundreds of houses, making it a distinct possibility.