The debris from the WTC is in a landfill in Staten Island.   Many portions were also removed and have become memorials in many local towns, as well as perhaps elsewhere in the country. I have heard some of the steel was melted down and used in the construction of navy ships.

I know it is easy to think that the steel melted, but that has not been proven - at least that I have found. There is evidence of aluminum melting, which would be common in a building fire that also involved an airplane.