Oyster-type travel card system gradually being rolled out across the entire Netherlands (and not just in the capital like Oyster is here). This will replace the familiar strippenkaart that you might have used on Amsterdam's public transport. There is a disposable "tourist" version that you can pick up in the same places that strippenkaarts are/were sold (they are both available at the moment).
Official and slightly bonkers (in a good way) Dan Rhodes website. Worth a diversion.