Piece of Heart

A classic Zelda item. Four Pieces of Heart are required to form one Heart Container in most games, although in Twilight Princess you need 5 to form 1 whole heart container. Pieces of Heart are scattered all across Hyrule and the other game loactions. Link can usually aquire a Piece of Heart by completing a task for an NPC, reaching a difficult part of the terrain to get to, or through many other means.

A Link to the Past

 * ZeldaUniverse.net's Guide
 * Karri Huismann's Guide

Ocarina of Time

 * ZeldaDungeon.net's Guide

The Wind Waker

 * ZeldaUniverse.net's Guide

Other games

 * Super Smash Bros.
 * Super Smash Bros. Melee