I have some ideas... Okay, I already put them in another thread, but that was before I knew that there was an official item thread (silly me...) :embarrass
Okay, here they are:
Cape Feather: Gives you a cape that allows you to slowly float to the ground after jumping/falling, by holding the jump button. (Super Mario)
Note Block: Wherever it's thrown, it will stay for a while, either in the air or at the ground. It will act as a stationary trampoline.
Bowser Shell: Like a normal green shell, but much bigger! (Mario Kart)
Magic Dust: Sprinkle it over an opponent to turn him into a harmless blob creature for several seconds. Can still jump and walk, but not attack or double jump. (Zelda)
Gust Jar: Used to suck opponents and/or items to you that are far away. (Zelda)
Ice Spreader: A gun that has only 3 shots, creates a huge field of ice upon impact. Everyone who is close by at that moment will be frozen. The surrounding ground will be slippery for a while. (Metroid)
Wave Beam: A gun that can shoot through walls and electrifies the enemy. (Metroid)
Nugget: Gives you a big amount of special points once you collect it. (Pokemon)
Poke Flute: One-use item. Use it and all opponents are forced to dance for a short while. (they can't do anything else) (Pokemon)
Lock-On Gun: Charge it to lock on to an enemy and release to fire a homing shot. (Starfox)
Nova Bomb: Thrown item, only explodes when it hits an opponent. Huge blast radius! (Starfox)
Warp Creatue (don't remember their name): Two of them appear at once. Once activated, you can warp between them anytime. (Yoshi's Story)
Metal Barrel: You can throw it and it bounces off walls. When it's rolling back to you, you can jump on to ride on it. (DK Country)
Tire: You can roll it around and use it as a trampoline. The enemy can use it as well. (DK Country)
Mike: You can yell into the mike up to three times, and every time you do it, the opponents that are close by are hurt and thrown back by the sound. (Kirby)
Yoyo: Allows you to attack with a long-rage yoyo for a while (Kirby, Earthbound)
Eggplant Wizard's Wand: Fires eggplants in an arc that turns the opponent into an eggplant creature for a short while (similar to Magic Dust, but as a long-range projectile) (Kid Icarus)
What do you say?