Snowball

Snowballs are a Feast of Winter Veil event item, made to remind everyone it is winter!

Source
During the Feast of Winter Veil, snowballs can be purchased for in stacks of five from:

A few snowballs are given as quest rewards for Feast of Winter Veil quests and can be looted from snow mounds on the ground in the Alterac Mountains area of Hillsbrad Foothills and across Dun Morogh.

During the Midsummer Fire Festival, snowballs can be obtained from the Pocket Full of Snow.

During the rest of the year, snowballs can be made by an engineer using a Snowmaster 9000 at a rate of one snowball per day.

Snowstorm can drop a Snow Patch which contains 1-3x Snowballs.

Use
Snowballs can be thrown at other players. There is a slight impact animation. In the past they used to do actual knockback, which could have major secondary effects (knocking a character off a high location).

Requires a snowball to use:
 * Winter Veil Disguise Kit - (snowman disguise)

Snowballs were also required to use the following, until patch 4.3.0:
 * Green Helper Box - (green Father Winter's Helper companion)
 * Red Helper Box - (red Winter's Little Helper companion)
 * Jingling Bell - (tiny Winter Reindeer companion)
 * Snowman Kit - (Tiny Snowman companion)

Each activation consumed a single snowball.