Ting is a popular MVNO in the US, known for consistently offering great deals. However it only worked on Sprint's CDMA network, until now. Recently, the company started testing a GSM service on an invite only basis, but has now opened it to everyone.
Technically, the service is still in beta, so a few features are not yet ready. But you can add a GSM phone to Ting's plans right now if you are interested. As with other MVNOs, you need to have an unlocked GSM phone to use Ting. Order the $9 universal GSM SIM X1 card and plug it into your device. Ting is using T-Mobile as its GSM network partner, so if T-Mobile has good coverage in your area, so will Ting. If you want to try Ting, there's an ESN/IMEI checker on the Ting site that will tell you if your device is supported.
You can also have CDMA and GSM devices on the same account. Once a phone number is activated on a Ting SIM card, the SIM is forever tied to that number. Thus, you can't port a different number to the same SIM later.
As we mentioned, not all features work as they should. For example, there's no international long distance or international roaming. However, these shouldn't be issues for most users. VoIP solutions are always available as alternatives, and given that your phone is unlocked, you will always be able to swap in a local SIM when you travel instead of relying on the roaming feature.
source - Ting