  We introduce an information free energy for thermodynamic systems driven by
external time-dependent parameters. We show that the latter is a nonequilibrium
state function and that it is a natural generalization of the usual equilibrium
and steady state free energies. We discuss its importance for the
nonequilibrium maximum work theorem and the Jarzynski relation in the presence
of feedback control. We further show that it is a nonequilibrium Lyapunov
function.
