In theories of semi-classical quantum gravity where the cosmological constant is considered a thermodynamic variable, the gravitational mass of a black hole has been shown to correspond to the enthalpy of the thermodynamic system, rather than the energy. The resulting extended thermodynamics now includes pressure and volume. This means that we can imagineer black hole heat engines that do useful mechanical work. We present a study of some of these in anti-de Sitter (AdS) spacetime. We also propose that the extension should be used for all spacetime solutions, and consider the case of Taub-NUT-AdS and Taub-Bolt-AdS geometries.