  We present a Monte Carlo simulation technique by which the free energy of
disordered systems can be computed directly. It is based on thermodynamic
integration. The central idea is to construct an analytically solvable
reference system from a configuration which is representative for the state of
interest. The method can be applied to lattice models (e.g., the Ising model)
as well as off-lattice molecular models. We focus mainly on the more
challenging off-lattice case. We propose a Monte Carlo algorithm, by which the
thermodynamic integration path can be sampled efficiently. At the examples of
the hard sphere liquid and a hard disk solid with a defect we discuss several
properties of the approach.
