FreedomPop is great for light-medium users looking for cheap (FREE) cellular data. They have offerings on Spring CDMA as well as AT&T LTE SIMs. The free allotment is not a lot, but sufficient for light users. You can always get additional SIMs (1 cent - 99 cents) and swap the out as needed or pay for additional data.
Just read the fine print and make sure to downgrade from trial plans/services before you get charged.