Sand

Sand is a block naturally found on coasts and Desert biomes mostly going 4 blocks deep until it transitions into Sandstone. Sand can be mined easily by hand, although using a shovel is faster, and gives resources regardless of the tool used. It has a very low block resistance.

Falling Sand
Falling Sand is an entity created when sand falls.

Behavior
Sand, if there is no block below it, will turn into Falling Sand and fall until it lands on the next available block, thus making it one of two blocks to which gravity applies (the other being gravel). This will happen even if the block of sand is attached to a Sticky Piston, as the piston will only stick to a block when retracting. When Sand is being affected by gravity and falling, it exhibits a smooth falling animation. If falling sand lands on a mob or the player and covers their head, it will suffocate them until they successfully destroy the block, move out of the block or die. If Falling Sand lands in the space occupied by a non-solid block, (such as Torches, Slabs, Minecart tracks, or Redstone), it will disintegrate into a sand resource block. If it falls onto a cobweb it will slowly fall until it has gone through it completely, or until it touches the ground, at which point it will turn into a resource block. Placing a block of sand into the space directly above a non-solid block will not cause it to turn into Falling Sand. Sand blocks can be suspended in indefinite defiance of gravity in this way, most famously using torches.

Current Uses
Sand can be used in the construction of Airlocks and the creation of mob suffocation traps. It is also useful as a scaffolding material in some situations. Being affected by gravity, it can be quickly and easily removed from ground level and can be used to construct a safe path downwards from a higher position (such as a hole directly down through the roof of a large cavern) even if the ground itself cannot be reached by hand.

Torch Mining
When mining a large column/pillar of sand, a good method is to dig under the stone or dirt that it is resting on. Under that dirt or stone, place any partial block (such as a torch or slab). Mine the dirt or stone and the column falls into the placed object, quickly destroying the sand. This can also be used to mine Gravel.

Cofferdam mining
If large quantities of sand and sandstone are needed the ocean floor is a great source of both of these. To create a cofferdam, find a place in a large lake or ocean that is only one or two blocks deep and fill in that spot to be just above water level. Then dig a pit inside so that the water can't get back in. With a large enough pit, a large chest can be filled with sand and sandstone fairly easily.

This method can also be used in Desert biomes without the need to drain away water.

History
Sand has been available for use since Classic Mode. During that time, Sand blocks were rare and only appeared naturally in one block-thick beaches (usually by water or in the middle of a landscape). These beaches were always level and below Ocean level. Sand instantly appeared at the lowest point it could go when it is when placed above an air block without any sort of falling animation. Destroying a pillar made from sand from the bottom would cause the sand to disappear from the top first. These attributes were present until the release of Infdev.

Map editors could be used to create floating Sand, although the server would crash if the Sand was affected from its state.

Before Beta 1.3, generated sand did not have Sandstone under it, and if a cave spawned below sand, then floating sand blocks would occur until one of the blocks was updated.

Trivia

 * In classic, if the player is next to a set of blocks and places 2 sand blocks above them, using the side of those blocks, they will end up inside of those blocks, and notice the internal texture is identical, but darker than regular sand. Like most blocks, they are also hollow. The same bug happened in gravel.
 * An old glitch in Classic mode allowed players to raise the height of a fluid block by placing Sand over it. The Sand would stay suspended in mid-air until it was broken. When broken, a fluid block corresponding to the type below the sand would appear where the block was. The suspended fluid block would remain immobile until a block was placed next to it, causing a flood.
 * If a block of sand has snow on it and is made to fall, the snow is destroyed and will not drop a snowball.
 * Before the Beta 1.8 pre-release, players would always spawn on sand in single-player Survival mode.
 * If you put a sand block directly above a torch, it will stay in the air.