Without any restriction on negative energy densities, general relativity allows exotic phenomenon such as the construction of time machines. Negative energy densities are possible, but in free field theory they are restricted by the averaged weak energy condition and the quantum inequalities. Here I exhibit a system of a domain wall coupled to a massless field, which has static negative energy density and thus violates all these conditions. The domain wall system has an advantage over the traditional Casimir effect with metal plates, because there are no uncertainties involving dispersion relations and surface roughness.