Mark Frauenfelder: Neatorama has an excerpt of "10 scientific frauds that rocked the world," from a book called Condensed Knowledge, by Mental Floss.
The Quadro Corporation of Harleyville, South Carolina, had an impressive client list: public schools, police agencies, the U.S. Customs office, and Inspector General’s offices to name a few. The product they sold, the top of the line Quadro QRS 250G (also known as the Quadro Tracker, available for $1,000), boasted the ability to find drugs, weapons, or virtually anything worth looking for. The small plastic box supposedly contained frequency chips of an advanced sort not known to regular science. Driven by static electricity, the Quadro would resonate at exactly the same frequency as the searched-for item.
When the FBI opened the box, however, they found nothing inside. Quadro threatened to sue Sandia Laboratories when Sandia suggested that the device was fraudulent, but eventually Quadro became the bigger company, and just closed shop.