Laszlo Hanyecz (laszlo) made the first documented purchase of a good with bitcoin when he bought two Domino's pizzas from jercos for 10,000 BTC. laszlo had made contributions to Bitcoin's source code in the past.
The Pizza Index is the value of the bitcoins spent on the pizzas, were they to be sold rather than spent on pizza. It started at $41 when the transaction actually occurred, and rose significantly since then, topping at $15.5 million in April 2017.
Bitcoin Pizza Day
To commemorate the transaction, May 22 is dubbed Bitcoin Pizza Day. Pizza providers worldwide offer discounts to bitcoin users to commemorate laszlo's purchase.