A small canister (usually 8 or 16g) of nitrous oxide gas (N2O). Used officially in making whipped cream, but more often used recreationally to for its dissociative effects. If used properly and in moderation whippits are harmless. However, improper inhalation (e.g. wearing a mask or putting a bag over one's head) can lead to oxygen deprivation, and heavy use can lead to vitamin B12 deficiency. You'd have to be a complete moron to put a bag over your head in any situation, and the B12 deficiency is easily treated, so nitrous is a pretty safe drug.
Me: I knew someone whose 19 year old son died by using whippits. Apparently he put a plastic bag of the nitrous over his head.
Someone else: What an idiot.
Me: Yep, there's always someone who gives a safe drug a bad name.