Term for special usable items?

These items are usually known as power-ups or buffs, depending on the situation.

In many arcade and older style games they are known as power-ups and are usually picked up during play, but in other game spaces(MMOs and the like) they are known as buffs, and are used with item or ability.

Negatives effects are pretty much the opposite naming. Power-downs (used very rarely and only in arcade and old games) and debuffs used everywhere else.

I would normally go with 'powerup', as (in my mind), 'buff' or 'debuff' usually refers to a status effect of some sort that doesn't necessarily involve picking anything up. 'Pickup' might also be an effect-neutral term that fits what you're looking for.