If a player reveals their hand in a multiplayer game, does everyone get to look at the player's hand, or only the controller of the ability that forced them to reveal?
You reveal the card to all players.
701.13a To reveal a card, show that card to all players for a brief time. If an effect causes a card to be revealed, it remains revealed for as long as necessary to complete the parts of the effect that card is relevant to. If the cost to cast a spell or activate an ability includes revealing a card, the card remains revealed from the time the spell or ability is announced until the time it leaves the stack.