The Plumbers Badge has the Omnitrix Symbol on it. It can be used to detect where other Plumbers or Plumber's Kids are and activate a device in the rust bucket that automatically adapts to many situations eg. spiked tires for ice movement, an front mounted object similar to a cow trap for protection while offroad down a mountainside and a sonar vision screen for finding an invisible Big Chill then proceding to shoot an electrified net at him which canceled out his intangibility and invisibility. It can be used as police badge to break up criminal activities that the Plumber would work on. Plumber's Badges were mentioned quite a lot in Ben 10 Alien Force episode Plumber's Helpers.

Owners Of Plumber's Badges
Ben Tennyson/The Omnitrix
Gwen Tennyson
Max Tennyson
Kevin E. Levin (his father's)
Magister Labrid
Magister Prior Gilhil
Alan Albright/Pyronite Kid (his father's)
Mike Morningstar/Darkstar/His Badge Was Destroyed
DNAlien Plumber's Kid
Possibly The Other Plumber's Kids