AT&T has the most coverage in the US, but T-Mobile would also be good choice in major metropolitan areas (their coverage is not as good off the densely populated areas). They are the two core US GSM providers -- the others are virtual providers that resell either AT&T or T-Mobile service under their own names.

AT&T just dropped their prices on their prepaid plans -- you can get 10 cents per minute with no daily charge, or $2 per day with unlimited calling.

There are AT&T stores all over the place, and they will sell you the SIM card for about $5 to $10, and you'll just need to add credit. Cards are available in $15, $25, $50 and $100 denominations which will give your number 90 days of service ($15 will get you only 30 days of service).


Remember here in the US we pay for both incoming and outgoing calls on mobile numbers, so if you're planning for a lot of talk time the $2 per day deal works the best.