I have an unlocked device that I use for international travel. When in Europe, I purchase a local SIM card for each country that I visit.

I would like to purchase a pre-paid SIM card for use in the U.S. Since this would not be my primary cell phone in the U.S., use would be minimal. Any suggestions for the most cost-effective strategy? Is there a provider that makes it as easy as the Europeans? I don't want any contractual obligations.
Thanks in advance.