The "Stingray" is a neat little device that fools a cell phone into connecting to it as though it were a cellular phone tower. Once connected, the Stingray can record the device's unique ID, monitor the device's traffic, and even triangulate the cell phone's position.
It's also questionably legal. In June, unsealed documents revealed that Florida police were caught lying about using information from a Stingray to obtain warrants. As Ars Technica reports, officers were instructed to "refer to the assistance as 'received information from a confidential source regarding the location of the suspect.'" They were also told never to refer to the Stingray in police documents and to re-submit warrant affidavits that referred to them, according to the ACLU.
Lies notwithstanding, we still don't know any more about Stingrays, and now the FCC wants to get involved.