AT&T and Verizon, but it's always best to check your zip code to make sure there's coverage in your area.
I did think of one caveat with the newer smartphones - when a friend and I travelled to West Virginia last year, my smartphone didn't have any service. If I'd had my older, "non" smartphone, I would've been better off. They run on different systems, so it really depends on where you'll be using it.
"" A little learning is a dangerous thing."-Alexander Pope