Many atomic-level 0 K thermodynamic properties of silicon (100) surfaces have been calculated from classical potential energy surfaces or from quantum ground-state Born-Oppenheimer surfaces. We present an easy and accurate computer simulation method for determining surface thermodynamic properties at high temperatures where the dynamics may be described classically. The method is illustrated with calculations of the surface free energies of the 2 x 1 and c(2 x 2) reconstructed (100) surfaces at T = 1000 K using the Stillinger-Weber interaction model for silicon. We then use an extension of our simulation method to determine the line free energies associated with steps. Enough examples of line free energies are given to enable us to calculate the equilibrium concentration of single and double steps as a function of the surface miscut angle. We also deduce the activation free energy associated with the Si adatom hopping over simple steps.