Leaf

A leaf was a component of plants and trees.

During the autumn, dead leaves would fall to the ground. People would then blow the leaves away with leaf blowers. In a dream, Rory Williams at first mistook the sound of the TARDIS materialising for the sound of one. (TV: Amy's Choice)

Uses
Tea plants had leaves, which were used to make tea. Raspberry leaf tea was good for the ailments of the throat. Tea leaves were also read by psychics to read the future. (TV: The Awakening, AUDIO: Casualties of War, TV: The Sun Makers, TV: The Curse of Clyde Langer)

Tobacco leaves were used in making cigarettes. (PROSE: History 101)

Leaves in plants and species
Some clovers had four leaves, and were considered lucky. (COMIC: Triskaidekaphobia)

Cadonwood trees, trees native to Gallifrey, had silver leaves. (AUDIO: Neverland)

The trees native to Fragrance also had silver leaves. (AUDIO: The Fragile Yellow Arc of Fragrance)

Time Trees, native to Hirath, had orange leaves. (PROSE: Genocide)

Poppito trees, trees native to Raxacoricofallapatorius, had reddish-brown leaves. (PROSE: The Monsters Inside)

History
While on a colony built entirely on one giant tree, the Eleventh Doctor and Amy Pond once used a giant leaf as a parachute when escaping the Pulpavores. (COMIC: The Evergreen Death)

The the floor of the TARDIS Cloister Room was covered in dried leaves. (TV: Doctor Who)

"The most important leaf in human history"
A large red ovate leaf, later dubbed "the most important leaf in human history", caused the meeting of Dave Oswald and Ellie Ravenwood. On a windy day in Blackpool in 1981, this leaf fell from its tree and flew into Dave's face. This lead him to almost get hit by a car, before being saved by Ellie. After they began dating, and then got married, he kept the leaf, a symbol of their love.

Ellie would later put the leaf in the first page of her book, 101 Places to See, which she later passed on to her daughter, Clara Oswald.

Clara, who eventually became a companion of the Eleventh Doctor, used that leaf to feed Akhaten, a planet-like entity that fed off peoples' souls, and therefore stories. The leaf represented the history of the family, and well as potential futures. The infinite amount of stories in that one leaf bloated Akhaten, and he imploded. The leaf was destroyed in the process. (TV: The Rings of Akhaten)

The Doctor returned the leaf to Clara after she entered his time stream; the leaf helped to come back into the world. (TV: The Name of the Doctor)

Clara Oswald also had a maple leaf pressed in the pages of 101 Places to See. After saving Clara from having her mind being downloaded by The Great Intelligence, the Eleventh Doctor discovered the book containing the leaf. He asked Clara about and she told him, "That wasn't a leaf, that was page one." (TV: The Bells of Saint John)

Behind the scenes
It is unclear whether or not the maple leaf is the same as "the most important leaf in human history". It might however simply be a production error which caused the leaf to change form.