I know Peperomia Rotundifolia and Prostrata will both hang off of ledges. They dont seem to have strong enough steams to hold the fat leaves up unless they have firm rooting points. I start them in my moss, then they grow up the back wall, and once they reach the top they turn around and go back down
I havent been able to find any, but I have also seen Dischidia Hirsuta grow down in other people's vivs.
Another down grower is called Dendrobium pugioniformis. It is an orchid and seems relatively easy to grow. I have one in a grow-out tank and I dont pay any attention to it care wise.