A conker was a children's toy made from a horse chestnut seed and used to play a game of the same name.
When the Second Doctor visited the Panjistri homeworld, he had a pair of conkers in his pocket. (PROSE: Timewyrm: Apocalypse)
The Second Doctor explained the concept of a space elevator to a confused Jamie McCrimmon by using a conker on a string as a demonstration. (AUDIO: The Great Space Elevator)
The Third Doctor had Venusian conkers. (AUDIO: Ghost in the Machine)
The game was introduced by the Seventh Doctor in 8th century Britain. (COMIC: Doctor Conkerer)
On Olleril, the Seventh Doctor used a conker on a string to hypnotise the Robert Clifton Celebroid. (PROSE: Tragedy Day)
The Eighth Doctor carried a conker in his pocket. (PROSE: Kursaal)
The Tenth Doctor carried a conker in his pocket. (PROSE: The Price of Paradise)
During a fight with Cybermen, the Tenth Doctor wished they would agree on settling it with a conkers fight. (COMIC: The Power of the Cybermen)
The Tenth Doctor compared Bannakaffalatta to a talking conker. (TV: Voyage of the Damned)