I will discuss the mechanism behind the remarkable properties of double perovskites like Sr2FeMoO6 that show half-metallic ground states with 100% polarization and a ferromagnetic Tc above room temperature. I will conclude with a broad overview of other remarkable properties that can be achieved by changing the transition metal atoms. Reference: O. Erten, et. al Phys. Rev. Lett. 107, 257201 (2011).