Amazing device on the Enterprise in Star Trek: The Next Generation which has the ability to create completely realistic holographic simulations of anything you can imagine, even to the point of giving the simulations real solidity (using complex matter manipulation and transporter technology) and even, in many cases, artificial intelligence. Some of the coolest ST:TNG episodes involved the holodeck. Then again, some of the worst episodes also involved the holodeck. If I had a holodeck, I'd go in and never ever come out.