Shield Breaker
This page refers to one of Marth's attacks (Marth uses the same attack in Brawl). For information regarding combos that break shields, see here.
The Shield Breaker is Marth's Neutral B move. It is functionally similar to Roy's Flare Blade. However, the Shield Breaker charges much quicker and does not deliver as much damage, knockback, or any recoil to Marth. It does also not yield an explosion, as the Flare Blade does when fully charged. True to its name, it can break any shield once fully charged.
In Super Smash Bros. Brawl the move has had a change in animation. Instead of an overhead vertical slash with a large arc area, it has become a single powerful stab. When fully charged in the air, it moves him forward quite a bit. The style used now is much more in kind with Marth's battle animations in the Fire Emblem games. This is to reference Marth's game.
However, any changes to damage and speed is not known at this time.
A fully charged Shield Breaker in the air can be used to aid recovery if it is performed when Marth is knocked in the very top left or very top right position of the screen/stage as makes him move forward quite a bit. After performing a fully charged Shield Breaker, Marth can still use his Up B to aid further aid his recover.