Great Deku Tree

The Great Deku Tree, is the 'Father of the Forest' charged with watching over the Kokiri who live in Kokiri Forest and the Lost Woods. He is an exceptionally large tree with a human-like face located in a clearing in the southern area of Kokiri Forest.

Ocarina of Time
When The Legend of Zelda: Ocarina of Time begins, the Deku Tree is suffering from a curse cast upon him by Ganon in an effort to gain the Spiritual Stone of Forest which is in the Deku Tree's possession. Knowing Link's destiny, the Deku Tree sends Navi the fairy to retrieve the boy and asks him to destroy the curse spider within him. Though Link defeats Gohma, it is too late to save the tree. Before he dies, the Deku Tree gives Link the Kokiri Emerald and tells him to seek out The Princess of Destiny at Hyrule Castle. After adult Link completes the Forest Temple, however, he discovers a little sprout, which grows into the Deku Sprout. It is this sprout who tells Link the truth about his past, and reveals that Link is not a member of the Kokiri, but rather a Hylian who was given to them by his mother, who died soon after. This is possibly the same tree Link comes across in The Wind Waker, and that the Koroks, the leaf-faced tree creatures, are evolutions of the Kokiris that the Great Deku Tree once fathered.

The Wind Waker
In The Legend of Zelda: The Wind Waker, the Deku Tree was once the Deku Sprout from Ocarina of Time and now lives in the Forest Haven, and gives Link Farore's Pearl after Link defeats a hoard of ChuChus which attack the tree and rescues the Korok Makar. He also gives Link the Deku Leaf.

Twilight Princess
There is speculation that The Great Deku Tree's stump is in The Legend of Zelda: Twilight Princess. It is located to the south of the Great Bridge of Hylia. The stump is fairly large, indicating it may have been a Deku Tree. However, it is unknown if this stump is The Great Deku Tree's or if it is the Deku Sprout's. Also, note that there are ChuChus in the Forest Temple in The Legend of Zelda: Windwaker, as well as underneath the stump near the Great Hylian Bridge in The Legend of Zelda: Twilight Princess.

Trivia

 * A theory about the original The Legend of Zelda is that the first dungeon, Level 1, is a dead Deku Tree.
 * The Great Deku Tree is not to be confused with The Maku Tree in The Legend of Zelda: Oracle of Seasons and The Legend of Zelda: Oracle of Ages.