Some Americans are apparently buying Vodafone NL You SIM which are preactivated on eBay.
http://www.ebay.com/itm/Vodafone-YOU...-/282438519767
So it covers a lot of countries in Europe and for $7 you can hit the ground running, sort of.
You first have to top up, which may or may not work with US credit card.
And the top ups aren't that attractive, €15 for 1 GB?