Just some ideas for items from games I liked.
Powerpunch (Ape Escape): Smack opponents with a close-range extending boxing glove for a limited time.
Catapult (Ape Escape): Sling small rocks across the stage in slight arcs for a limited time.
Bee Mine Glove (R&C: Size Matters): Throw out small bee hives. Opponents that come near them are swarmed with bees.
Plasma Whip (R&C: Up Your Arsenal): Wield a plasma whip for a limited time, lashing opponents at a medium range.
Post your own if you have any.
Awaiting: Guild Wars 2, Bioshock: Infinite, The Last of Us, God of War: Ascension, Playstation All-Stars Battle Royale.
Boots of Hermes (God of War);
Grants the player a speed boost for the next 10-15 seconds
Car Keys (Gran Turismo);
Once picked up there will instantly be the sound effect of when a car is bought from Gran Tusimo. A Nissan Skyline drops down from the top of the screen where the player is stood, instantly killing any opponents that it lands on and then the car drives off, running over opponents and making them drop a bit of their Super meter as collectible orbs. This would be a rare item.
TNT Crate (Crash Bandicoot);
If the crates is hit then it explodes and instantly kills any opponents within it's proximity. If a player jumps on top of the crate then the classic 3 second timer count downs and it automatically explodes.
Nitro TNT Crate (Crash Bandicoot);
Similar to the ordinary red TNT crate, but this green TNT crate instantly explodes and kills players from just coming into contact with it.
Checkpoint Box (Crash Bandicoot);
The box must be broken with an attack. The player that breaks the box will instantly repsawn back at that spot without any respawn delay.
Playstation MOVE controller;
Changes your character's moveset like the Spear of Destiny to just 3 attacks, one for each button. One attack uses it like a whip, actually creating an energy whip out of the end of the orb on the controller. The second pretends to use it like a Bow 'n' Arrow, actually firing an arrow projectile across the screen. The third attack pretends to use the controller like a sword, creating the aura of a blade out of the end for a single strike. You can keep using this item until you finally successfully hit someone (if they block it doesn't count) and it causes the controls of a hit opponent to randomly screw up for about 10 seconds, but you lose the item.
Red Materia (Final Fantasy VII)
Works like the Pokeball from Smash Bros., but randomly summons one of many different summonings from Final Fantasy VII
Nanotech Box (Ratchet and Clank);
Break the box and a load of Nanotech orbs fall out that can be picked up to give the player some Super meter. I thought that this was a good idea considered the orbs that characters drop in this game look just like the Nanotech orbs anyway.
PS1/PS2 Memory Card;
All players, include the one who uses the Item, has their Super meter reset to zero.
Precursor Orb (Jak & Daxter);
Gives your a full bar of Super, but is an extremely rare item and doesn't stay on the stage very long.
Causes the your Super meter to switch around with whoever has the highest amount of meter. If you already have the highest then the player with the 2nd highest amount of meter will instantly lose meter until they have the lowest.
Uka-Uka (Crash Bandicoot);
Uka-Uka flies after a radomly chosen opponent. if he hits then he'll go on their face and take control. For 10 seconds the A.I. will control that opponent and will work alongside of you. You're not able to hit that player, nor is that player able to hit you. The controlled player won't gain meter from their attacks whilst under the influence of Uka-Uka and they cannot pick up orbs dropped by opponents, but Uka-Uka can't use their Supers.
Black Materia (FFVII);
Small meteors randomly rain down around the stage for 5 seconds.
Gravity Boots (Ratchet and Clank);
Gravity is drastically lowered for all characters except you.
Origami (Heav Rain)
For the next 10 seconds nobody can use their Supers. One player is unable to use any attack buttons. One player is only able to use one attack button, although which one he/she can use is randomly decided. One player can use all 3 attack buttons, but just only the neutral versions of them. One player can use all 3 attacks buttons just fine. These 4 effects are split amongst the 4 players. It's radomly decided which player ends up with which effect.
PSABR; Kratos, Jak & Daxter , Sly Cooper