The Booo-merang is an invention created by Fenton Works. The Booo-merang locks onto a ghost's unique ecto-signature and seeks it out wherever it goes. It is powered by an internal power source that can last for over Ten years.
Later, Jazz found that it wasn't locking onto whom she thought was Danny, leading her to figure out that it was really Dark Danny in disguise. In an attempt to get a message to Danny, trapped in the Ghost Zone ten years in the future, she writes a note to him and attaches it to the Booo-merang and sends it through the Fenton Portal into the Ghost Zone.
The Booo-merang flies through the Ghost Zone for ten years, until it once again detects Danny's ecto-signature and locks in on him. Because the note is written in Jazz's handwriting and attached with her headband, Danny learns that Jazz knows about his secret.
In "Kindred Spirits," Sam and Tucker use the Booo-merang to find Danny after he was kidnapped by Vlad. When it reaches Vlad's mountain chalet, it hits Vlad directly in the head, distracting him just as he was about to attack Danny and Dani, saving them.
The Booo-merang looks like a metallic boomerang, with green details and a green dial in the center of it. Sam once described it as a "stupidly named tracking device."