Do you mean like T-Mobile's Even More Plus or Flexpay?
Verizon has Month to Month, just select it in the drop down box (1 year, 2 year, Monthly) on their website. You obviously have to pay full price for the phone. Boost Mobile's month to month $50 unlimited plan is another option. AT&T's Go-Phone is $50 for unlimited Talk And Text.
Most are No Contract (Prepaid) besides Verizon and T-Mobile.