The Snowball is a Vanilla minecraft item. They can be obtained by using a shovel to break the thin layers of snow found covering blocks in snow biomes.
You can also create Snowballs automatically by trapping a Snow Golem on a single block and then putting a Block Breaker one block above the block the Snow Golem is standing on. By attaching that to a redstone clock or a RedPower 2 Timer, the Block Breaker collects the Snowball and the Snow Golem's ability to place snow wherever it stands, regenerates the Snow and the Block Breaker breaks the new Snow, it then gets regenerated an unlimited amount of times. The Snowball could then be pumped into a Compressor ( or Singularity Compressor) and the resulting Ice could be pumped into a Nuclear Reactor to cool it.
Snowballs can be compressed into an ice block, which is considered very useful for cooling down nuclear reactors. Aside from having the ability to be compressed into ice blocks they can also be thrown at other players or mobs, causing knockback but no damage.
Ice can be macerated into snowballs.
Water Cells can be compressed into snowballs.