Ice Beam

The Ice Beam is a weapon that appears in all Metroid games except for Metroid Prime 2: Echoes, in which it was replaced with the Dark Beam. It shoots a beam that has the power to freeze most enemies.

When frozen, certain enemies can be shattered with missiles (or charged shots in the Prime series). One notable case of this is the Metroid in hatchling form.

In Metroid Prime
When charged up, the Ice Beam can sometimes destroy enemies on contact. The missile combo for the Ice Beam is the Ice Spreader, which is a missile with a concussive blast of ice. It is very effective against flying enemies.

In other games
Samus has the power to stand on frozen enemies in all of the 2D games. This is usually crucial at some point in the games.

In Metroid Fusion, the Ice Beam is incompatible with the Fusion Suit because the Metroid Vaccine gave Samus a vunerability to ice, so she does not recover it until near the end of the game from the SA-X. Before that point, freezing is done with the Ice Missiles.