Pendant of Virtue

The Pendants of Virtue are three major Quest Items that appear in A Link to the Past.

Quest Purpose
The pendants are required in order to retrieve the Master Sword. Link must travel to three dungeons and collect each one before going to the Lost Woods to find the Master Sword. An ancient Hylian verse on the pedestal of the Master Sword about the pendants reads:

"The Hero's triumph on Cataclysm's Eve Wins three symbols of virtue. The Master Sword he will then retrieve, Keeping the Knights' line true."

Pendant of Courage
The Pendant of Courage is located in the Eastern Palace, close to the cave that Sahasrahla is hiding in. Its guardians are a group of six Armos Knights.

Pendant of Power
The Pendant of Power is found in the The Desert Palace, in the south-west of Hyrule. The dungeon bosses are three Lanmolas. Oddly, this Pendant is blue in color, while other items associated with power or Din (such as Din's Pearl or the Goron's Ruby) are colored red. However this could be because variants of stronger enemies are usually colored blue, such as in The Legend of Zelda.

Pendant of Wisdom
The Pendant of Wisdom is located in the The Tower of Hera atop Death Mountain. The guardian of the final Pendant is a single Moldorm. Like the Pendant of Power, this pendant is a different color than other items similar to it (such as the Zora's Sapphire or Nayru's Pearl).

A Link to the Past comic
In the A Link to the Past comic, Link must retrieve the three Pendants of Virtue to wield the Master Sword located in the Lost Woods. The Pendant of Courage is given to the young hero by Sahasrahla after he deems Link worthy of searching for the legendary blade. Link then finds the Pendant of Power hanging on a scorpion statue in the Desert of Mystery, and finally the Pendant of Power in the Tower of Hera by seizing it from an eye of a giant spider dwelling within. With the three pendants in hand, Link travels to the Lost Woods and claim the Master Sword to defeat Agahnim with it.

Theory
Some have noted the similarities shared by the Pendants of Virtue, the Goddess Pearls and the Spiritual Stones. This could indicate a direct link between these artifacts, or equally it could simply be an example of the designers' desire to create a feeling of familiarity or a sense of a "shared legend" within the series.