  Entropic forces in classical many-body systems, e.g. colloidal suspensions,
can lead to the formation of new phases. Quantum fluctuations can have similar
effects: spin fluctuations drive the superfluidity of Helium-3 and a similar
mechanism operating in metals can give rise to superconductivity. It is
conventional to discuss the latter in terms of the forces induced by the
quantum fluctuations. However, focusing directly upon the free energy provides
a useful alternative perspective in the classical case and can also be applied
to study quantum fluctuations. Villain first developed this approach for
insulating magnets and coined the term order-by-disorder to describe the
observed effect. We discuss the application of this idea to metallic systems,
recent progress made in doing so, and the broader prospects for the future.
