We talked today about how an atom entering an open T apparatus will be in a superposition of the three T base states. We wrote: |S+> = c_+|T+> + c_0|T_0> + c_-|T_->. Those states, or the paths they represent, interfere with each other so that the atom leaves the apparatus in the same |S+> state.
It sometimes happens that two particles interact so as to enter a superposition in which the spin state of one depends on the spin state of the other. Neither is in a definite spin state by itself. These particles are said to be entangled, and they can remain entangled over long distances. (This has been tested for separations of many meters, but in principle there's no limit.) Griffiths pp. 421-422 discusses some of the "spooky" consequences of entanglement.
Recently researchers have been demonstrating entanglement in macroscopic (roughly, human-sized) systems. Usually on this scale the effects of entanglement are washed out by uncontrolled interactions with the environment. Here's an article describing a sweet experiment in which physicists, wielding lasers, managed to entangle two chips of diamond. The diamond chips were entangled for all of 350 femtoseconds. (That's 350 * 10^-15 seconds, for those counting.)