Western Union do a pre-paid Mastercard that can be used wherever Mastercard is accepted...

https://wumt.westernunion.com/info/osPrepaidMC.asp

American Express does pre-paid cards in the form of gift cards, $50, 100 or 500, accepted wherever AmEx is accepted...

http://www10.americanexpress.com/sif...ection=cardnav

Visa Buxx has already been mentioned and looks like a good service and will be accepted wherever Visa is accepted.