The phenomenon of superconductivity could be extremely useful -- if the materials that exhibit the behavior could do so at ambient conditions. The first material discovered to conduct electricity with no resistance was mercury in 1911, but mercury requires temperatures below 10 °K to do this. In 1986, a high temperature superconductor was found that seemed to work around liquid nitrogen temperatures. We've made some progress pushing the limits of the superconductors
we've made so far, and it looks like we may be on the cusp of a much better understanding these materials and how they work. Here are just a few links on the matter.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post