  Thermodynamics teaches that if a system initially off-equilibrium is coupled
to work sources, the maximum work that it may yield is governed by its energy
and entropy. For finite systems this bound is usually not reachable. The
maximum extractable work compatible with quantum mechanics (``ergotropy'') is
derived and expressed in terms of the density matrix and the Hamiltonian. It is
related to the property of majorization: more major states can provide more
work. Scenarios of work extraction that contrast the thermodynamic intuition
are discussed, e.g. a state with larger entropy than another may produce more
work, while correlations may increase or reduce the ergotropy.
