A laser saw was a piece of technology owned by Torchwood Three. It appeared to be for surgical purposes.
The device could be used to open flesh with a laser with virtually no mess. It could also reattach it with no visible scar. When opening flesh, it would show a red beam, and when closing flesh, a blue one.
In 2009, Captain Jack Harkness and Ianto Jones used the laser saw to open up a dead body and retrieve a seemingly harmless alien from inside him that released endorphins into the body. A few hours later, Agent Johnson took it from Harkness and opened up his stomach to implant a small but powerful bomb, while he was temporarily killed by her. The bomb later detonated, destroying the Captain and the Hub. (TV: Children of Earth: Day One)