You know those MetroCards that have a random, unusable amount of money left on them, that are destined to burn a hole in your pocket? If you take all the leftover money on those things, it adds up to $52 million a year. So why not have a kiosk that allows you to donate your card's leftover funds into one central fund, which is then donated to charity? That's the idea behind MetroChange.
The physical card would be taken for recycling, and the money on it would go to charity. The creators (students of NYU's Interactive Communications Program) they have researched how to make this happen (in nerd-speak: "There is a kiosk with Arduino, ethernet shield, LCD screen, IR transmitter and receiver, and magstripe reader"), and that each person donating would be able to choose on the kiosk which charity to donate to.
You can go through their testing process right here, where they conclude, "We’re continuing this project—we hope to find more ways of eating into these odd problems with the MetroCard system upstream. After that we’ll devise ways of using any remaining value that is left on cards in a more productive way." We've reached out to the MTA, who may have something to say about all this, and will update when we hear back.