Check out Straight Talk. I have been using them for two years or so now. They are a MVNO meaning they provide the service using other carriers. ST uses Verizon, AT&T's and in a few places T-Mobiles network to provide the service. Here in my area they have both Verizon and At&T based phones. The plans they have are pretty good. With most phones you have a choice of either a $35 plan with 1000 min. of talk, text and 30MB of data or they have a $45 unlimited plan. They have 2 smartphones in their lineup that only run on the $45 plan but the phones can be hard to get because of their popularity. You can order the phones from their site or get them from Wal-mart.http://www.straighttalk.com/
Type in yor zip code to get a list of phones. Phones ending in "C" run on Verizons network and phones ending with "G" run on AT&T's or T-Mobiles network. There are two smartphones not listed but are available at Walmart when in stock are the Nokia E71 and the Nokia 6790. These both run off AT&T and require the $45 plan. I have the E71 and it's a great phone for the price of the plan. Cost of the phone is a bit high but you pay more for the phone because there is no contract.