You can put the sim card from your phone into any that is 'locked' to the same provider. There are programs that you can download on the internet that 'unlock' GSM phones to be used with any provider. Some generate codes that you type into the phone, some require cables.
You can find auctions on ebay too, that claim to email you an unlock code when you win the auction. I have unlocked 2 nokia phones that I had with cingular and my sister uses one with T-mobile just fine.