  The accurate determination of transport coefficients in numerical simulations
is becoming increasingly important in a wide range of applications. Here we
consider the linear response in systems driven away from thermal equilibrium
into a non-equilibrium steady state with non-vanishing entropy production rate.
A simple derivation of a general response formula is presented. For Markov
processes we explicitly construct the conjugate quantities and discuss their
relation with known response formulas. Emphasis is put on the formal analogy
with thermodynamic potentials and some consequences are discussed.
