On 22nd May 2010, programmer Laszlo Hanyecz made the first documented purchase of goods using bitcoin when he exchanged 10,000BTC for two pizzas supplied by Jeremy Sturdivant (which he had bought from Papa John's). Back then - when the technology was just over a year old - that equated to roughly $25 per pizza.
Here is a link to the original "Pizza for bitcoins?" post on bitcointalk.org.
Here is a link to the transaction on the Bitcoin blockchain.