Belt of Giant Strength +6

Increases the Strength of the Hero by 6 when worn.

Source
Cairne Bloodhoof can find one in The Oracle, chapter 7 of the Warcraft III orc campaign, The Invasion of Kalimdor.

Kael, Lady Vashj and Illidan can find one in Gates of the Abyss, chapter 5 of the Frozen Throne alliance campaign, Curse of the Blood Elves.