Istanbul's smart ticket is known as an Akbil, and it works on buses, trams, the metro, and most ferries. It's a small disk on a piece of plastic that you can hang on a key chain; it's useful if you plan to travel a lot by public transport, as fares are slightly cheaper with it, but the main draw is convenience. The initial purchase includes a deposit of about $4—you need the receipt to get the deposit back, so hold on to it. The Akbil can be bought and refilled at all major transport hubs.