Sea sponges are very simple animals that live on the ocean floor. They attach themselves permanently to an anchorage, and move sea water through their bodies, filtering out tiny organisms for food. The channels that the water flows through account for their hole-riddled structure, and is what makes their composition so useful.
They are harvested by divers; sponge-diving has been a family tradition in many areas around the Mediterranean Sea and off the coast of Florida in the U.S. They have been used as cleaning tools for thousands of years.
Manufactured cellulose sponges have decreased the market for natural sea sponges, and for common household cleaning use, they are certainly sufficient and much cheaper. Still, there are some applications where you might want to spend the extra money to get the genuine article.
If you love your car, you might want a large natural sea sponge to wash it with. When wet, sea sponges are very soft, much softer than their artificial cousins, and much less likely to damage a finish.
Some women prefer the gentle texture of a sea sponge to apply makeup, and remove it. Artists have always used sea sponges, both in clean-up of their tools and as another way to apply paint to a surface. Home decorators are now often 'sponging' paint on walls to create a particular look, such as a faux marble or stone.
One of the most interesting recent (and more than likely ancient) applications of sea sponges is as natural tampons. Several lines of natural bodycare products now offer sea sponge tampons, which are nothing more than sea sponges of a particular size. The sea sponge is soaked in water, squeezed as dry as possible, and then inserted into the vagina, where it absorbs the menstrual flow. If it is uncomfortable, the user can simply trim away some of the sponge to get a more comfortable 'fit'. The sponge can be removed and rinsed and reinserted every few hours until the user's cycle has ceased. It can then be cleaned with vinegar, or peroxide and air dried before being stored for future use. When cleaned properly, a sea sponge used as a tampon can last six months or more, and is much more earth-friendly than disposable tampons. They also do not carry the risk of Toxic Shock Syndrome that regular tampons pose.
Sea sponges are a renewable resource; unless the oceans become too polluted for them to live, we can count on a steady supply.