Snowball

The snowball is an item that was added during the Minecraft Alpha 1.0.5 update. They are 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, but is abled to knock back mobs and gives the same effect as if it was damaged. However, they still seem to damage a wolf, 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.

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

Traps

 * Snowball pitfall trap:
 * 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

 * Before the Seecret Saturday Update 1, you could get more than four snowballs from each block.
 * Throwing a snowball through lava causes it to catch fire resulting in a flaming snowball which will still do no damage nor set things on fire.
 * When riding a pig with a saddle, it is possible for the player to "fly" by throwing snowballs down at the pig.
 * Like arrows and eggs, snowballs can be fired from Dispensers.
 * In SMP, when throwing a snowball at somebody, the player getting hit by the snowball will absorb it with no visual effect or damage.
 * The fireball that ghasts shoot are actually flaming snowballs. This, however, will be changed in Beta 1.9.

Future
In the upcoming 1.9 Adventure update, a craftable mob, known as a Snow Golem, will have the ability to attack hostile mobs with snowballs.