The Legend of Zelda: Oracle of Seasons/Heart Pieces

There are 12 Heart Pieces to be found. Every four pieces make an extra Heart Container, for a total of three.

Heart Piece in Horon village
Hop down the ledge just south of the mayor's house, go up the steps, and burn down the small tree with an Ember Seed to get your Heart Piece.
 * Chapter 2: Horon village
 * Requirements: Seed satchel & ember seeds (found in the 1st dungeon)

Heart Piece in east suburbs
Go out the eastern exit of Horon Village (south of the Woods of Winter) and then head east one more screen. Hop on top of the stump and change the season to winter. Head north one screen and west one screen, and enter the cave. Inside is a Heart Piece.
 * Chapter 2: East suburbs
 * Requirements: Rod of Seasons & Power of Winter

Heart Piece in Woods of Winter
In the Woods of Winter, find the pond; lift the rock with the power bracelet and claim the heart piece.
 * Chapter 3: Woods of Winter
 * Requirements: Power bracelet (found in the 2nd dungeon)

Heart Piece in Subrosian market
Link can buy the Heart Piece in the market for the price of 10 ember seeds plus 10 ore chunks.
 * Chapter 3: Subrosian market
 * Requirements: 10 ember seeds and 10 ore chunks

Heart Piece from Maple
Maple appears every time Link defeats 30 enemies (every 15 enemies, if he wears Maple's ring). Occasionally, Maple will drop one Heart Piece if you hit her. She will go for that item first, so you need the Pegasus Seeds to get faster than her. The Pegasus seeds tree is found in chapter 3. The Heart Piece appearance is completely random, so it may take a while to get; Maple's ring will give you more chances at it.
 * Requirements: Pegasus seeds & Maple's ring (not mandatory, but they help a lot)
 * Chapter 3: anywhere, after Link gets Pegasus seeds.

Heart Piece in Moblin's Keep
With the help of his animal friend, Link can reach the western entrance of the Great Moblin' Keep. After defeating him, a chest holding the heart piece can be found in the ruins.
 * Chapter 4: Moblin's Keep
 * Requirements: Animal friend's flute

Heart Piece from a Gasha nut
There are four Gasha spots where one Heart Piece can be obtained:
 * Chapter 4: Goron Mountain
 * Requirements: shovel
 * 1) Goron Mountain base (accessible from chapter 4)
 * 2) Eyeglass lake, near Unicorn's Cave (accessible from chapter 5)
 * 3) Mount Cucco, in autumn (accessible after chapter 5)
 * 4) Tarm ruins (accessible from chapter 6)

Heart Piece on Mount Cucco
On Mount Cucco, change the season to spring. Then, use a "spring flower" to be thrown on an upper ledge. Go east a few screens, then jump down near a tree stump.
 * Chapter 4: Mount Cucco
 * Requirements: power of Spring

Heart Piece in Spool Swamp
Go to the tree stump near the entrance of Poison Moth Lair (dungeon 3), and switch the season to spring. Follow the river all the way south, and the current will bring Link onto this Heart Piece.
 * Chapter 4: Spool Swamp
 * Requirements: power of Spring & Zora flippers

Heart Piece in Eyeglass lake
In the Eyeglass Lake area, switch the season to summer, swim through a small stretch of water, then explore the eastern basin. Lift a rock and explore the mini-dungeon (three rooms) to get the Heart Piece from a chest.
 * Chapter 5: Eyeglass Lake
 * Requirements: Zora flippers & power of Summer

Heart Piece in the graveyard
After clearing the seventh dungeon, get to the tree stump on the west coast. Using the Roc's cape, go back to the graveyard and collect the Heart Piece using the power bracelet.
 * Chapter 8: Graveyard
 * Requirements: Power of autumn and Roc's cape

Heart Piece in the temple remains
After Link made the Subrosian volcanoes erupt (using a bomb), this room becomes accessible. Blast the entrance with a bomb, then use the Roc's cape to reach the heart piece.
 * Chapter 8: Temple Remains
 * Requirements: bombs and Roc's cape