It's called the Levitron. But who cares what it's called, because it's simply awesome. It uses a technique called spin stabilized magnetic levitation to keep the top stabilized in mid-air, giving it the illusion of "anti-gravity." Now, since I don't know how magnets work, I'm not going to talk about how it actually functions. Alternatively, you could check out their page which explains the physics behind the Levitron by clicking here.