When I'm in France I can use my VISA card for purchases but the old style magnetic strip cards have been all but phased out over there and replaced with "chip and pin" cards that are a great deal safer. Anybody with $20 in their pocket can buy a machine to read the data off a magnetic strip card. Most establishments and all ATMs in France will still read the magnetic strip cards but as time goes on it gets more difficult to use them. On occasion they'll be refused and other times the cashier just doesn't know what to do with them. The last thing you want when you're in a country that doesn't speak your language is to have a cashier asking you questions while 20 angry customers wait for you to stammer. However, I've always been able to work around all of this by taking out enough cash for the day and using it as backup if my card doesn't work.
However, there is one situation where cash and a magnetic strip card won't help - Velib. Velib is a very cool bicycle renting system in France. Every 300 ft or so there's a station with a ton of Velib rental bicycles. You rent the bike and ride it to your destination then turn it back in. The first 30 minutes is free so if you're just going from place to place you may never pay anything. Or you can just keep the bike and pay by the hour. However they only use Chip and Pin credit cards keeping me on my feet. That is until now.
Travelex, the money changing people have introduced what they call the Chip and Pin Passport which is effectively a prepaid VISA card that has a chip in it just like the European ones.
To purchase one you got to a Travelex store and load the Chip and Pin Passport Mastercard with the amount of Euros or Pounds Sterling that you need. After your trip if you want to reclaim any remaining money you return to the store and they'll give you cash back. I'm not sure where Travelex is making their money but considering their bussiness is buying and selling money I think they view this prepaid cash card the same way they view money exchange - you're buying foreign currency from them, they're just giving it to you on a card. I'm sure the cost is in that process and not the card itself.
Visit the Travelex website for details.