The finite-temperature magnetism of ultrathin films such as sesquilayer Fe/W(110) and submonolayer Fe/Cu(111) is investigated. Based on renormalizations of Onsager's exact solution of the two-dimensional Ising model it is shown that superparamagnetism is a common phenomenon in imperfect ultrathin films. The ultimate reason for this behavior is the existence of two structural length scales: the lattice constant, usually considered in renormalization-group theory, and the characteristic size of the film inhomogeneities.