Mobile Virtual Network Operators don’t maintain their own network infrastructure - instead they lease access at wholesale rates from the big four (Verizon, Sprint, AT&T & T-Mobile). They market their own brands and provide their own customer support.
MVNOs are common abroad, and are gaining momentum in the US. They provide access to the same networks without contracts, and at cheaper rates.
The catch is that you need to provide your own smartphone, but you can obtain one easily:
Some providers allow your existing number to be ported across, but we recommend using Google Voice to manage your phone number.
Once you’ve ported your number to (or chosen a new one from) Google Voice, you can switch prepaid providers at will with ease. You could even try a different one each month - the switching cost is very low when you’re not on contract.