A number of airlines like British Airways or Emirates run a program in partnership with a charity. The principle is simple: you have spare change in the country you're coming from that you haven't or couldn't change back into your home currency when at the airport. Rather than bring it back home and use it as paperweight, you just drop it into an enveloppe and the airline does what's necessary to get the money to their chosen charity.
British Airways normally runs a program called Change for Good, in partnership with UNICEF. But, for the month of January 2005, all funds will be given to the relief effort in aid of the victims of the Asia quake disaster. So if you're flying BA this month, drop your change in the enveloppe, they accept any currency and every little helps.
This idea of asking international travellers to donate their spare change is in my opinion a great idea. At the end of the day, when you are coming back from a country you are not likely to return to anytime soon, all spare cash is just dead weight anyway, especially coins because you won't be able to change them back. So giving them to charity is the obvious solution and being able to do it on your flight home just makes it very easy. In my experience, only a small number of major airlines do it. I reckon they should all do it. It's easy, they've got the captive audience on the plane, it wouldn't take much.