An electronic device that essentially simulates the distress call of a large (usually Alliance) ship -- it makes a fine distraction when you need to make an Alliance cruiser decide between chasing you or rescuing what they think is a big ship full of people.

It is, we will assume, HIGHLY illegal for any of a dozen reasons.

Serenity's crew uses such a device (Cry Baby #6) to good effect in the Serenity Pilot episode. Point of trivia: it's duct-taped to a Blue Sun coffee can.