It can be anything (Held item, medicine, battle items, berries) Template:
>Item name
>Type
>Effect

I'll start.

Leaf Charm
Held item
An item to be held by grass-type Pokemon. If struck by a one-hit kill move, it will survive it and its Speed, Attack and Sp. Attack will increase sharply for one turn (single use only).

Caffeine Pills
Battle item
Prevents the Pokemon from sleeping and increases speed by one stage for 5 turns.

Cucum Berry
Berry
In a pinch, the power of all super-effective moves will be doubled.

Legendary Feather
Held item
Increases the attack power of Flying-type moves by 75% but reduces the power of Grass, Ground and Fighting moves by 25%.

Mithril
Held item
Increases the attack power of Steel-type moves by 75% but reduces the power of Rock, Poison and Ice moves by 25%.