Muon-catalysed cold fusion is the only plausible method of room temperature fusion. It uses a subatomic particle called a muon, which can be thought of as a heavier electron, to replace electrons in the atoms of deuterium or tritium. The muons are considerably heavier than electrons (207 times, to be precise), and by replacing the electrons they cause a proportional decrease in the size of the atom. This makes it easier for the atoms to come together and fuse, making room-temperature fusion a possibility.
Unfortunately, muons have to be created in a particle accelerator and are unstable, usually decaying in a few milliseconds. They are also attracted to the helium atom produced by the fusion, preventing it from catalysing more fusion. These problems mean that muon-catalysed cold fusion has yet to become efficient enough for commercial use.