There are ATMs practically everywhere in Europe and most students found this to be the easiest way to get cash. However, before you leave, speak to your bank about (1) the high fees they may charge for overseas use and (2) not flagging your account as fraudulent due to the overseas use.
To counter the large fees some students took out large amounts of cash at one time. If you choose to do that be careful with your cash. (watch out for pickpockets!)
Western & Northern Europe:
Used in most places.
Eastern & Southern Europe:
Many places do not accept credit cards.
Those that do more widely accepted MasterCard and Visa over AMEX.
Some students came with them as backup for their cash and credit cards.
Do not rely on them as they are not always accepted.
If you lose them or if they get stolen you can put in a claim for your money.
as of February 2013
$1.34 = 1 Euro €
Exchange rates change daily, so for the most accurate information look at: http://www.xe.com.