What cell phone companies are gsm?

What cell phone companies are GSM? and is it true that if you buy a phone, and get it unlocked you can use it for any company? thanks.

Sorry I'm not sure what gsm stands for..
However yes that is true, I think it is with most phones. You'll have to check from the shop you buy it from, most new models will apply to this though.

The GSM Association maintains a list of phone companies operating GSM networks:

http://www.gsmworld.com/roaming/gsminfo/cou_us.shtml

AT&T and T-Mobile are on the list, Sprint and Nextel are not. Generally speaking, American phone companies lag the rest of the world in adoption of GSM, having invested a lot into CDMA infrastructure in the early days of cellular telephony...

As to unlocking the phone, yes, you can use it with any company that supports the phone's communication protocol (CDMA-only phone won't work on a GSM-only network and vice versa). You may or may not be able to use services other than basic telephony though, as those often require a special chip and/or software that phone companies ask phone manufacturers to install.