People have yearned to speak with the dead through a variety of methods throughout the ages, including using Ouija boards and psychic mediums. In this day-and-age of modern science, we have an electronic device that purportedly allows ghosts to audibly communicate with the living. This device is called the Ovilus or a Puck, and its use in paranormal investigations is controversial.
The Ovilus gained widespread notoriety when the cable show Paranormal State used it in an episode. To explain the device simply, the Ovilus measures electromagnetic waves in the surrounding air. EMF or electromagnetic frequency meters are commonly used by modern day paranormal investigators who believe an increase in electromagnetic activity indicates paranormal activity.
What is really intriguing about the Ovilus is not that it measures electromagnetic waves in the air, but what the device does with those measurements. The Ovilus actually vocalizes words using a database of vocabulary terms. What word the Ovilus says depends on the electromagnetic field readings the device detects. I don't want to get too much into the technical details of how the device works, but that is the high level overview of its operation. The principle is that ghosts can influence the electromagnetic field in a room, so a ghost might change the reading so the Ovilus says the words the ghost wants to communicate with the living.
This device is considered highly controversial and is marketed for entertainment purposes only. Some paranormal investigators have reported hearing intriguing and telling messages using the Ovilus. Other groups have discontinued using the Ovilus because they have received random or nonsensical messages from it. Really, though, people have received mixed results from Ouija boards as well, so this shouldn't be much of a surprise. Oh, and most Ouija boards (like the ones made by Parker Brothers) are marketed as novelties or toys and not as a way to actually communicate with the dead.
If you would like to see the Ovilus in action, check out this video from Paranormal State: