Making a negative-index material that works for visible light is more difficult, because the building blocks have to be much smaller--no bigger than 10 to 20 nanometers. That's now very possible to achieve, however, and several groups are working on it. If it can be done, these metamaterials could be used to increase the amount of information stored on CDs and DVDs or to speed up transmission and reduce power consumption in fiber-optic telecommunications.
We can also concentrate electromagnetic fields--the exact opposite of what the cloak does--which might be valuable in energy-harvesting applications. With a suitable metamaterial, we could concentrate light coming from any direction--you wouldn't need direct sunlight. Right now we're trying to design structures like this. If we could achieve that for visible light, it could make solar power more efficient.