I picked up a set of practice Butterfly Swords, steel ones, not the lightweight aluminum ones.
These are really fun! They come in pairs, one for each hand, and are pretty heavy for their size. Using them is an upper body workout in itself!
They are not a common weapon system in most martial arts. They come from a Southern Kung Fu tradition. I've only seen reference to them in Hung Gar and Wing Chun. Here is a Wing Chun form I found.