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 for a stack of five from:

A few Snowballs are given as quest rewards for Feast of Winter Veil quests.

During the Feast of Winter Veil, Snowballs can be looted from snow mounds on the ground in the Alterac Mountains.

During the rest of the year Snowballs can be made by an engineer using a at a rate of one Snowball per day.

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).

Snowballs are also required to use: Each activation consumes a snowball.
 * - (green Father Winter's Helper companion)
 * - (red Winter's Little Helper companion)
 * - (tiny Winter Reindeer companion)
 * - (Tiny Snowman companion)
 * - (snowman disguise)