Tenth Doctor's hand

The Doctor's hand was the severed right hand of the Doctor which he lost shortly after he regenerated into his tenth incarnation.

During a sword fight with a Sycorax leader on the deck of their ship, the leader slashed his right hand off and it fell somewhere in London. Because he was within the first fifteen hours since his regeneration, he was able to regrow the hand. (DW: The Christmas Invasion)

Captain Jack Harkness traveled from 200,100 to 1869 using a Vortex Manipulator. After living through the entire 20th century, he found the hand and used it to detect the Doctor whenever he came to the Cardiff Rift to refuel, hoping to find a version of the Doctor which coincided with his own timeline.

After traveling to the year 100,000,000,000,000, the Doctor, Martha, and Jack met Professor Yana who then became the Master. He took the Doctor's hand from Jack and escaped in the Doctor's TARDIS after he regenerated. (DW: Utopia)

The Doctor was able to fix Jack's Vortex Manipulator so they could travel to 2008. When they encountered the Master, he revealed that he put Richard Lazarus' genetic technology into his laser screwdriver. By using the Doctor's biological code he gained from the hand, the Master was able to age the Doctor 100 years. (DW: The Sound of Drums)

What followed were the events of The Year That Never Was, after which the Doctor reclaimed his severed hand and kept it in his TARDIS. (DW: Last of the Time Lords)

Just as the Doctor and Donna were saying goodbye to Martha after the Sontaran invasion of Earth, the TARDIS set off by itself. (DW: The Poison Sky) The TARDIS had brought the trio to the planet Messaline. The Doctors hand had detected that he had a daughter Jenny. Jenny was created when the hand that the Doctor had regrown was put in a Progenation Machine, thus creating Jenny and attracting the TARDIS, creating a paradox. (DW: The Doctor's Daughter)