  We compute Casimir forces in open geometries with edges, involving parallel
as well as perpendicular semi-infinite plates. We focus on Casimir
configurations which are governed by a unique dimensional scaling law with a
universal coefficient. With the aid of worldline numerics, we determine this
coefficient for various geometries for the case of scalar-field fluctuations
with Dirichlet boundary conditions. Our results facilitate an estimate of the
systematic error induced by the edges of finite plates, for instance, in a
standard parallel-plate experiment. The Casimir edge effects for this case can
be reformulated as an increase of the effective area of the configuration.
