Snowball

The Snowball is an item that can be obtained when a snow block (4 snowballs obtained) or snow (1 snowball obtained) is destroyed with a shovel (as of the Alpha 1.0.5_1 update). Snowballs can be placed in stacks of up to 16. They are a throwable item that does not do any damage to Mobs, with the exception of Blaze (which they deal to) and Enderdragons (which they deal  to), but is able to knock back mobs and gives the same effect as if it was damaged. However, they still seem to damage wolves; whether this is a glitch or not is currently unknown. This makes it useful for herding, and for knocking creepers off high areas. Snowballs cannot be placed and can only be thrown. Four snowballs can be combined in order to create a snow block.

History
Snowballs were added during the Alpha 1.0.5 update. Before the Seecret Saturday Update 1, you could get more than four snowballs from each block.

As a Crafting Ingredient
In Beta, if 4 snow balls are made into a snow block which you place and then break, the Player will not be able to reclaim it under normal conditions. Instead, the Player will reacquire the four snowballs.

In Minecraft 1.0.0, a new mob created with Snow Blocks and a Pumpkin known as a Snow Golem has the ability to attack hostile mobs with snowballs.

Also in that same release, two new mobs by the name of Blaze and Enderdragon, take damage from snowballs, making them the only mobs that Snowballs can currently be used to kill.

Traps

 * Snowball pitfall trap:
 * (Please note that this trap is currently ineffective, due to a bug which prevents dispenser-launched projectiles from dealing knock-back.)
 * Load a dispenser with snowballs and aim the dispenser at a connected pressure plate by a hole. If something walks across the plate, the dispenser shoots a snowball to knock the intruder into the pit, where they can be dealt with however you like.

Trivia

 * Throwing a snowball through lava causes it to catch fire resulting in a flaming snowball which will still do no damage or set things on fire.
 * Like arrows and Eggs, snowballs can be fired from Dispensers.
 * The fireball that Ghasts shoot were actually flaming snowballs. This, however, was changed in the first Beta 1.9 prerelease, and subsequently Minecraft 1.0.0.