[rainfreak]

You could probably jailbreak it and use it with another carrier IF the owner hasn't reported it stolen. The real question is how would you feel if you lost your phone and someone else found it and didn't return it? I personally would be very bummed out, so therefore I wouldn't do that to someone else. Instead, I would look for a contact for "home" or "mom" or "dad" and call one of them and tell them that I found the phone, and then arrange to give it back to them. You could also walk into any AT&T store and return it and I am certain they would ensure that the rightful owner got the phone back.

That said, it's up to you...