There are several cheap calling cards available. Have a look at
www.cheapphonecards.com -- I have used both the Mega Clean Pinless and the Voyager Pinless, and both were cheap to Europe and easy to use. You could combine these with a T-Mobile To Go prepaid SIM (calling Europe from TMO otherwise costs $1.50 a minute). AT&T Go Phone is overall, not as good as TMO, but its international calls are cheaper (I think 36 cents/min to Europe) -- but still too expensive for other than occasional use.
Be aware that many phone cards charge extra for calling from a payphone, and that some phone companies charge astronomical international rates from landlines unless the user has ordered an "international package." So be sure to ask first before placing international calls from a friend's house. OTOH, the two calling cards I mentioned have a toll-free number, so you can call from someone's home phone without worrying that they'd be charged.