The Booo-merang is an invention created by Fenton Works. The Booo-merang locks onto a ghost's unique ecto-signature and, once it is thrown like a regular boomerang, seeks that ecto-signature out wherever that ghost goes until it finds its target. It is powered by an internal power source that can last for at least ten years during active use.
Later, Jazz Fenton 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. By this point in time, the Booo-merang is visibly weathered, but remains functional and still has the note attached to it. Because the note is written in Jazz's handwriting and attached with her headband, Danny realizes that Jazz knows about his secret.
In "Kindred Spirits," Sam Manson and Tucker Foley use the Booo-merang to find Danny after he was kidnapped by Vlad Plasmius. 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 metal boomerang, with green details and a green dial in the center of it. Sam once described it as a "stupidly named tracking device."