Researchers at Duke University and Imperial College have created the world's first "cloaking device", a metamaterial ring that hides a copper piece from detection by microwaves. While still a far cry from the fictional technology known to Star Trek fans, this is the first experimental demonstration that metamaterials with a negative index of refraction can in fact be used to obtain this kind of effect (which had been theoretically predicted).
A metamaterial is a lattice-like arrangement of structures that acts like a material with electromagnetic properties not found in conventional materials (no, they have nothing to do with lattice QCD at all, but I liked the catchy caption). A negative index of refraction occurs in metamaterials whose effective electric permittivity ε and the magnetic permeability μ are both negative; in this case, the refractive index n=ε1/2μ1/2 becomes negative, and waves are refracted the opposite way as usual. For more on this pretty interesting stuff, see here and here, or (for people with appropriate subscriptions) here.