I adore this MIT project, which aims to make spending easier - or harder. Affixed with a Bluetooth module, the wallet connects to a mobile phone and the user's online banking, so it expands when rich and deflates when poor.
That's just one of the designs, the Peacock, but there are two others which are equally innovative. The Bumblebee buzzes and vibrates every time a transaction is made, with the pricier the purchase, the longer the buzz.
Mother Bear is most likely the most frustrating for big spenders. It's got a hinge inside, which clamps the wallet shut when the person has gone over budget (it can be programmed with the month's budget maximum). As that maximum approaches, the wallet becomes harder to open - you could say the user is a tight wallet.
While the bulk of the components aren't ideal, it's obvious that the idea behind them is superb. The MIT team hopes they can scale the technology down to something the size and shape of a credit card - though I should think it'd have to be implanted pretty firmly, to stop the user from taking it out and stomping all over it. [MIT via DesignBoom via GizMag]