There are a few truly free plans and they are currently offered by FreedomPop and RingPlus.
There is the Truly Free 2 Plan offered by RingPlus and the 100% Free Plan from FreedomPop
For truly free plans make sure not to accidentally sign up for any premium services or added value features by accident during the sign up or activation process
Here are all the plans available from FreedomPop including the free plan
Here are the RingPlus plans including the free plans
There are a few other “free” plans offered as well, but there is a Top-Up fee meaning a cost that will activate in case the customer were to go over the amount of talk, text, and/or data allowed. Once the customer goes over and the Top-Up is activated there will be a charge for another Top-Up.
FreedomPop also offer Free Talk, Text, & LTE Data for a Year but you will need to buy a specific phone for $69.99 (Feb 2016)