2 Answers

Here is a list of all the carriers that use GSM technology in the US:
Airfire Mobile
ASTAC
Asset/Vada Wireless (uses AT&T/ TMobile)
AT&T (Includes GoPhone Prepaid, Dobson Cellular, Edge Wireless and Centennial Wireless)
Broadpoint
Calhan Wireless
call4care
Cellular One of East Central Illinois
Cellular One of East Arizona
Cellular One Nation
Cellular One TXOK
Chariton Valley Wireless
Cincinnati Bell Wireless
Commnet Wireless (also uses CDMA)
Consumer Cellular (uses AT&T towers)
Cordova Wireless
Corr Wireless
Cross Communications
DTC Wireless
Earthtones
Epic PCS
Fuzion Mobile
GCI Wireless (also uses CDMA)
GTC Wireless (uses AT&T towers)
i wireless
Immix
Indigo Wireless
Jolt Wireless (uses AT&T towers)
KTC PACE
Locus Mobile (uses CDMA, Verizon & AT&T towers)
Long Lines Wireless
Mobal Freedom (uses AT&T towers)
NEP Wireless
Pine Cellular
Plateau Wireless
Pure Prepaid (uses AT&T towers)
Pure Talk USA (uses AT&T towers)
Simple Mobile (uses T-Mobile towers)
Shaka Mobile (also uses CDMA, Verizon & Sprint towers)
Telecom North America Mobile Inc
TerreStar
T-Mobile USA
TracFone Wireless (also uses CDMA, AT&T, Verizon, Sprint, US Cellular and T-Mobile towers)
Tru (uses T-Mobile towers)
Union Wireless
Viaero Wireless
Wal-Mart Family Mobile (uses T-Mobile towers)
West Central Wireless
Westlink
XIT Communications

Read more: http://cellphoneforums.net/general-service-provider-forum/t349388-list-us-gsm-carriers.html#ixzz2RAPLtzFg

I would suggest you to visit http://www.forsi.it/node/85  Here you can find GSM carrier in all countries.If you want to choose GSM carrier in particular country means,Make sure your phone is unlocked and accept that carrier.If you have a locked phone means,You will need to unlock your phone.For unlocking use http://www.theunlockarena.com/supported-phones/rs13wp7/  When i chose my GSM sim card service,I followed the same way.So only i recommend this one.I hope my post is very useful for who are need GSM sim card service.