What cell phone service providers can I use with an unlocked iPhone 4?

I was thinking of unlocking my iPhone. My two year contract with AT&T ended and the iPhone 4 I have doesn't have service with them anymore. If I were to unlock it, what cell phone companies would be compatible with the iPhone? [Which are cheapest?]

Added (1). Would virgin mobile work?

Any providers that use SIM cards - T-Mobile is the biggest one. Which other ones you can use depend on where you live.

Any GSM carrier.No, Virgin Mobile would not be one of these as it is CDMA. Others include, Sprint, Verizon, metroPCS, Boost Mobile, Cricket, etc.

Your phone can be used with any gsm network.
http://attiphoneunlocker.com/

You can get a free sim card for simple mobile at my site for free. We have othr companys to but simple iis the best seller.

Then you got the iphone and jailbreak the i phone then which you save the contents. In SIM and phone mammary you have the two options. In next time used one options always. That are undertake the security. And after one mouth you can upload the contact file in our PC. And i hope that you will not miss the contact details.