The Home-run Bat (ホームランバット, hōmu ran batto) is an item used in all three games and is almost always an instant KO when smashed, thus like the Hammer, making it one of the most "feared" items in the game. The Home-run Bat is an item used in the Home-run Contest to knock the Sandbag as far as possible. It originates from the Smash Bros. series according to the trophy description, though many believe it should hail from the EarthBound series.
When used in a Smash Attack, it does around 30% damage and almost always instantly KO's the opponent with a distinctive pinging sound except in the largest possible custom stage where with proper DI and heavy weight it can be survived, but Donkey Kong can Spinning Kong momentum cancel without help of DI at all. It cannot be charged and always has a somewhat long attack animation where the character sparkles.
In Brawl, this animation is unique from a character's standard battering item smash attack animation, most of them resembling more traditional baseball bat swings.
When dropped, it does 16-18% damage.
When thrown at the opponent, it does between 19-22% damage with high knockback.
In Home Run Contest, the tip of the bat makes the sandbag fly farther. This is referred to as a "tipper".
Also note that in Super Smash Bros. Brawl's Home Run Contest, at 390% or higher a smash up throw is stronger than the tipper forward smash bat, making it useful in Co-op Home Run Contest without characters with a strong enough attack to be stronger than the bat.
Hitting a member of the Subspace Army (other than bosses) with a Home-run Bat will cause them to fly very far, useful for knocking them off if their health is higher than the 30% damage the bat can deal. This strategy will not work if a wall is blocking them from flying away, or if the enemy in question is immune to flinching (a la Autolance or Floow).
A battering item. Smash a foe with this and be rewarded with the distinctive "KREEEEENG!" sound of a home run as your foe goes flying. The sound echoes with what's surely the timbre of supreme bliss. For the character being hit, though, it must be one of the scariest sounds ever. But it's not easy to use-- it swings slowly and has a limited range. High risk, high reward.