Magna-Clamps were metal devices found by Torchwood One. When attached to an object, it cancelled the object's mass, enabling it to be easily lifted. They were capable of lifting two Imperial tons even if only held in one hand. Magna-Clamps were not available for public use. They were activated by pressing a button on the handle.
Torchwood One found the Magna-Clamps buried in a crashed spaceship at the foot of Mount Snowdon. They were then removed and taken back to Torchwood Tower. Yvonne Hartman showed the Magna-Clamps to the Tenth Doctor and Jackie Tyler during their tour (Jackie says that she could use them to carry her shopping bags), (TV: Army of Ghosts) and later on during the Battle of Canary Wharf the Doctor and Rose Tyler used the Magna-Clamps to hold onto a wall when the Void was open. This stopped them from falling into the Void until Rose let go and almost fell in. (TV: Doomsday)
A set of Magna-Clamps was present in the UNIT Black Archive. (TV: The Day of the Doctor)