Ring of Regeneration

Increases the Hero's hit points regeneration by 2 hit points per second.

Source
Arthas and Muradin Bronzebeard find one in Dissension, chapter 8 of the Warcraft III human campaign, The Scourge of Lordaeron.

Arthas can also find one in Fall of Silvermoon, chapter 5 of the undead campaign, Path of the Damned.

Thrall can find this in The Oracle, chapter 7 of the orc campaign, The Invasion of Kalimdor.

Tyrande can find one in Brothers in Blood, chapter 5 of the night elf campaign, Eternity's End.

Kael and Lady Vashj can find one in A Dark Covenant, chapter 2 of the Frozen Throne alliance campaign, Curse of the Blood Elves.

Kael and Lady Vashj can find one in The Crossing, the secret level of that campaign.