Joseph Shoer of Cornell University has built a levitating model spacecraft to demonstrate how a tractor beam could potentially work in real life. In the video, Shoer uses a "ludicrous" neodymium magnet to manipulate his non-ferromagnetic model which floats on air. Says Shoer of his experiment:
The [tractor beam] would work by pumping changing magnetic fields at a target spacecraft, exciting eddy currents in the spacecraft's aluminum skin. These currents interact with the magnetic field from the tractor beam device, allowing it to push, pull, or rotate the target. In the video, I generate these changing magnetic fields by moving a big rare-earth magnet around. On a spacecraft, a more likely tractor beam device would be a set of electromagnet coils.
[Via Quantum Rocketry]