Here are the two largest.
Green Splendour (Luffa cylindrica)
a.k.a. Loofah, Dishcloth Gourd, Vegetable Sponge Gourd
Slender, dark green sponge gourd. Harvest for eating at 9-12" long. Plants require a lot of space, so plant seeds in hills spaced 6 feet apart or space plants about 3-4 feet apart in a single row. Trellis vines for straight fruit. Long vines.
Seeds from Johnny's Seeds
The fruit of L. aegyptiaca may be allowed to mature and used as a bath or kitchen sponge after being processed to remove everything but the network of xylem.
How to prepare the loofah for a sponge
Plants are ready to harvest just before the vine dies. Pick pods from the vines and clean them lightly with warm water.
For complete instructions in transforming your loofah into a beautiful sponge go to eHow