Can you use a iphone with any service?

Found a iphone, with no data on it (contacts, pictures, nothing) and its not activated. I wish i would return it but i don't know where to start. I have ATT but what if its a sprint iphone? Or Verizon? I don't know what to do. Keep and use it (always wanted a iphone, or pawn it )

Keep it. Get it unlocked and use with any prepaid services. If you need help with getting the phone unlocked, mail me for references.