V. Pieces of Heart

In a small room through a Bomb-able wall in a cave in the northwest of Kakariko Village.  There are two entrances; only by dropping down into the hole will you be able to reach the Bomb-able wall.

The hut to the west of Sahasrahla's house in the north of Kakariko Village apparently used to belong to a gang of thieves.  The basement of this hut has a weak north wall that can be Bombed to reveal a small room with a Piece of Heart.

If you visit the house in the extreme south of Kakariko Village, you'll find a boy who has sealed the door to his brother's room.  Dash Attack or Bomb the weak wall to reach the other end of the house, and exit.  There is a woman outside who will ask you to navigate the maze there in 15 seconds or less.  Do so, and you'll win the Piece of Heart.  (Hint: There is a sign in the middle of the maze; if you make a beeline for it and jump down at that point, you should make it in less than 15 seconds easily.)

In the south of the Swamp Ruins, there is a small building that you can enter (the equivalent of the Dark World's Flooded Palace).  Inside is a chest with Bombs, but you won't need them to get the Piece of Heart.  Make your way to the north door, where you'll be in another room with a Bubble and two pull-switches.  The right-hand one releases the floodgates to the right, causing the trench to fill.  Go back outside, and you'll find that the water around the building has drained, revealing a Piece of Heart.

In the Desert Sanctuary, in the room where you talk to the old man, there is a weak south wall.  Bomb it to find a small room with a chest containing the Piece of Heart.

After exiting the Desert Palace for the first time (through the southwest exit), head south along the narrow ledge to find a Piece of Heart (watch out for the vulture).

After buying Zora's Flippers, head south over the waterfall, and swim southwest until you find a grassy passage to the west, which has a Piece of Heart at the end of it.

There is a pile of 5 rocks on a plateau just to the west of Sanctuary; Dash Attack it to open the entrance to a cave containing a Piece of Heart.

South of Spectacle Rock, in the cliff-face, there are two caves.  The right-hand one leads to a room with a Piece of Heart.

The most obvious Piece of Heart in the game, of course, is right on top of Spectacle Rock.

While traveling through the Lost Woods, you may happen across a group of 9 bushes arranged in a square.  The center one hides a hole which, if you drop down it, lands you right near a Piece of Heart.

Upon entering the Dark World after defeating Agahnim, you'll find yourself on the Pyramid of Power.  Go down a couple of levels, and head east until you see an otherwise inaccessible ledge below you.  Jump down and head east and north to reach a Piece of Heart.

In Hyrule, northwest of the entrance to Death Mountain, are twin lumberjacks cutting down a tree that they say seems strange to them.  After defeating Agahnim, return there to discover the lumberjacks missing, but the tree still there.  Dash Attack the tree to reveal the entrance to a secret cave, which has a Piece of Heart inside.

In the Dark World's Frozen Lake, there is a circle of stones northwest of the Ice Lake Cavern.  Stand in the circle and Mirror to Hyrule to find a Piece of Heart on an otherwise inaccessible island.

South of the Dark World's Haunted Grove is a circle of bushes.  Stand in the center and Mirror to Hyrule to find yourself on a plateau; go inside the cave here to find a Piece of Heart.

In the north of the Dark World's Graveyard is a ladder leading up to a small, empty plateau.  Mirror from there to Hyrule, where there is a cave with a Piece of Heart.

In the northwest of the Village of Outcasts is a house with a treasure chest on the sign above the door; this is the "Treasure Chest Game".  Inside you'll be able to open two chests for 30 Rupees.  Among the prizes you can win are Rupees (up to 100), Arrows, Bombs, Hearts, and one Piece of Heart.  It may take several tries to find the Piece of Heart, but it's nowhere near as frustrating as the next one.

In the southwest of the Village of Outcasts, in the place where the "15-Second Game" would be in Hyrule, is the Treasure Field.  (Aaaarrgh.)  If you pay the man there 80 Rupees, he'll loan you a shovel and allow you to dig in the field as much as you want for 30 seconds.  It could take several (dozen) tries to find the Piece of Heart, and can be very frustrating.  My suggestions: don't collect anything you dig up except for red Rupees, four of which will compensate for what you paid to get in.  In addition, don't dig in one place for long; keep moving around, to cover as much ground as possible in the time allotted.

At the Dark World equivalent of the entrance to Hyrule's Death Mountain, there is platform above the cave entrance with a Piece of Heart, plain as day.  Inside the cave, head up the north stairs to enter a room with a gap in the floor; use the Hookshot to grapple the pots on the other side and cross the gap.  Head south, and you'll find a blue bumper blocking your path.  Use the Magic Cape to walk straight through the bumper, and out the south exit to reach the Piece of Heart.

South of the Dark World's equivalent to the Blacksmith's Shop is a field full of purple stakes. Hammer every single one of them down to turn the boulder in the south of the field into a cave entrance. Inside the cave you'll find a Piece of Heart.

In the northeast of the Swamp of Evil, there is a cul-de-sac which, if you Mirror to Hyrule from it, will land you on a plateau with a boulder.  Lifting the boulder will reveal a cave entrance, inside of which is a Piece of Heart.

There are two caves on either side of the entrance to Misery Mire; the west one contains a Piece of Heart.

On the summit of the Dark World's Death Mountain, just to the east of the bridge, is a boulder; lift it to reveal a cave entrance.  Inside, head north along the gantry until you come to a dead end.  Use the Ether Medallion to reveal the path ahead of you, and cross the gap to the north door.  In the next room, there are two weak walls; the north one leads to a Great Faerie Fountain, and the west one leads to another room with two weak walls.  The north wall here leads to a Faerie Fountain, and the south wall is the exit.  Outside, you'll find yourself on an isolated island above Death Mountain (…what??).  Mirror to Hyrule to find a Piece of Heart waiting for you.

During the journey through Turtle Rock, you'll briefly exit the dungeon and cross a narrow ledge outside in order to reach the east cave entrance, beyond which is the Mirror Shield.  Before entering the cave, stand in front of it and Mirror to Hyrule to reach a totally isolated ledge with another cave.  Inside the cave you'll find a chest with the last Piece of Heart.