Chameleon Arch

The Chameleon Arch was a piece of Time Lord technology which could modify the biology of a Gallifreyan so the cells registered as another species. The Chameleon Arch was composed of a headset, a fob watch-like device used to store the Gallifreyan biological information, and an added perception filter. (DW: Human Nature / The Family of Blood)

Function
The Chameleon Arch changed every cell in the user's body. It was hooked up to the TARDIS' databanks to provide both physiological and historical information. It used electrical signals to change the cells. The procedure was very painful, though the reversal was quick and painless (as the body was simply returning to normal). (REF: Doctor Who: The Visual Dictionary)

Some traits of the user's true Time Lord heritage remained: John Smith was able to deliberately cause a complex chain reaction to save a mother and her baby from being crushed by a piano. Smith also kept a dream journal in which he chronicled memories of the Doctor's adventures. (DW: Human Nature/The Family of Blood) Professor Yana heard drums beating in his head all his life. (DW: Utopia)

Notable uses
There was another account suggesting the Doctor, prior to his regeneration into his eighth incarnation, made use of a broken Arch to create the fiction that he was half-Human, to use against the Master. (IDW: The Forgotten, referencing events depicted in Doctor Who)

The Tenth Doctor used a Chameleon Arch to change himself into the human, John Smith, in order to elude the Family of Blood. (DW: Human Nature/The Family of Blood)

The Master also used the Arch to turn into a human to escape the Last Great Time War, and remained in human form until he opened the watch and remembered who he was. (DW: Utopia)

The Doctor's Chameleon Arch was also used to help the Tenth Doctor recover his memories, after his mind was taken over by Es'Cartress of the Tactire. (IDW: The Forgotten)

The Tenth Doctor also briefly considered that Jackson Lake might be a future incarnation of himself under the influence of an Arch. Lake did possess what turned out to be just an ordinary fob watch. His initials at the bottom revealed his identity to himself and to the Doctor. (DW: The Next Doctor)

Known Chameleon Arches
After the destruction of Gallifrey, there were only two known Chameleon Arches left in the universe. One belonged to the Doctor inside his TARDIS, though he gave the fobwatch to Timothy Latimer. He seems to have either recovered it or had duplicates, in light of its later appearance in the Eleventh Doctor's drawing room. (VG: TARDIS). The other was in the possession of The Master before he left the end of the universe to the 21st century. (DW: Human Nature, Utopia)

Behind the scenes

 * Although only shown to remove all memories from the Doctor and the Master, the Doctor believes Jackson Lake to have put his memories into a fob watch. As the Doctor knew that Lake was still able to remember that he was "the Doctor", it implies that specific memories can also be put inside them, as well as all of them.