The 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 when ever he came to the Cardiff Rift to refuel.

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)