While traveling I've had a few companies charge me in a currency other than dollars on my credit card. A few have tried to change the exchange rate in their favor on me (ebookers[dot]com -- don't use them). To fight back you need to get the exchange rate from the day it was charged to your card. To do this go to this website:
Note that you can also get current exchange rate (cannot specify the date) on google by typing something like "1 dollar to euro" in the search box without the qoutes.