Christine's Sniper Rifle. I didn't try New Vegas until much, much later than the rest of you, so I didn't play the DLC in released order - I played them in the order I found them. Old World Blues, Broken Hearts, Dead Money, Lonesome Road. When I got the sniper rifle, it wasn't really that impressive, so I just dumped it in the sink, and forgot about it.
Hours, and hours later, I finally got to meet Christine, and we went through a physical manifestation of hell together. Once we'd cleared it out, she decided to stay there, for the rest of her life, watching over the Sierra Madre, and there was nothing I could do. Nothing. So I went back to the sink, and took her sniper rifle with me. If she won't leave the Sierra Madre, then I can at least take a little piece of her with me instead.
Also, you can use it to shoot things.