Bracer of Agility

Increases the Agility of the Hero by 1 when worn.

Source
Arthas and Jaina find one in Ravages of the Plague, chapter 3 of the Warcraft III human campaign, The Scourge of Lordaeron.

Malfurion, Tyrande, and Maiev can find one in Balancing the Scales, chapter 5 of the Frozen Throne sentinels campaign, Terror of the Tides.

Arthas, Kel'Thuzad and Sylvanas can find one in King Arthas, chapter 1 of the scourge campaign, Legacy of the Damned.