The first space shuttle orbiter, the Enterprise:
On display (at least for the time being) at the Smithsonian National Air and Space Museum’s Udvar-Hazy Center, the Enterprise never really had much of a chance to fly to orbit. It was built for air-drop and ground testing in the early days of the space shuttle program. The original plan was to later retro-fit it for flight as the second operational orbiter, but it wound up being easier and cheaper to just build a new one from scratch.
Now Enterprise’s future is (no pun intended) a bit up in the air — as the shuttle program wraps up its last flights, various museums are jockying to lay claim to the orbiters as they are retired. Most likely the NASM will get first pick, meaning that the Enterprise will need to move elsewhere to make room for it. So should you be passing through the Washington DC area, you might want to take the opportunity to see the orbiter before it heads off to parts unknown.