The Flipper is a projectile item in Super Smash Bros. Melee, originating from the NES game Balloon Fight. After it travels a certain distance, it stops in midair. When hit, it spins either slowly or quickly (depending on either the speed of someone who has contacted it, or the strength of the attack), blocking passage and causing light (though generally repeated) damage when touched. Aside from causing general havoc in a fray, they can also be placed strategically at integral locations, such as ledges or against walls. This will prevent opponents from recovering, or not allow them to pass through certain points. The downfalls of this item are that it can be avoided with relative ease and has a relatively short duration on the field.
When used on moving stages (such as Big Blue, Rainbow Cruise, Poké Floats, etc.), the flipper will continue to move with the background instead of remaining stationary. This is most apparent on Big Blue, where the flipper hangs still for a split-second, then races off screen as the Falcon Flyer and racers speed away from it.
In Melee, the Flipper seems to have replaced the Bumper from the original Super Smash Bros., as their usages and effects are quite similar. The Bumper's return and Flipper's absence in Super Smash Bros. Brawl further supports this theory, especially as the Bumper's new ability to be set in midair is very similar to the Flipper's. It could therefore be considered that after formerly being replaced by the Flipper, the Bumper then in turn replaced the Flipper for Brawl.
The flipper comes from Balloon Fight. In Balloon Fight, if a player bumps into a flipper, it starts to spin. If a player or enemies hits the flipper while it's spinning, the person flies temporarily out of control. After awhile, the flipper stops spinning. In Melee, the flipper can be picked up and thrown. When it is thrown, it stops in mid air. When a player attacks the flipper, it starts to spin. If a player is to touch the flipper while it's spinning, it knocks the player away. Also, the flipper slowly stops spinning after a certain period of time.