Luigi throws a fireball out in front of him. The fireball only goes straight forward, unlike Mario's fireballs. It isn't very fast and doesn't do much damage.
Up + B
Super Jump Punch
Luigi uppercuts an enemy, and if an enemy is vary close to Luigi, he'll do a fieryuppercut with 25 % damage. If not, the enemy only gets 1%. Unlike Mario's/Doc's Up + B, this move only goes straight up.
Down + B
Luigi Cyclone
Luigi spins around rapidly, sending enemies every which way. This attack can be used for recovery if the player is tapping the B button very, very fast.
Forward + B
Green Missile
Luigi flies forward, this move can be charged to go more forward. It's a 12,5 % chance of the Green Missile being a Spontaneously Misfire. The Misfire makes Luigi go even more forward than usually.
This trick requires two Luigis standing within range of their up-B attacks, but so that their up-B does not connect with its fiery sweet-spot. At this point, both Luigis can alternate using their up-B attack, climbing higher and higher into the air; they will accumulate damage but never die. This tactic can be used as a stall in team tournaments and is usually banned.