Conventional store-bought shower curtains are made from polyvinyl chloride (PVC), a petrochemical product that is toxic in its manufacture and use (it outgasses a suspected carcinogen and hormone disrupter). It is being phased out of toys and household products in Europe, and hopefully North America will soon follow suit. Because it is relatively inexpensive to make, it's found in a huge variety of items. Read more about the dangers of PVC here.
Alternatives to vinyl shower curtains are available commercially in the form of fabric curtains. Most are also made from petrochemicals, however. Hemp and organic cotton shower curtains are available, but expensive.
I decided to make my own eco-friendly shower curtain out of a large piece of gauzy 100% cotton from my fabric stash. All I had to do was hem it to the appropriate length and add twelve grommets to the top. Because it's so lightweight, it dries quickly and doesn't attract mold. It's easy to wash, and can be dried by re-hanging it while still wet. It looks great in my bathroom, adding to the natural spa effect of white tiles and fixtures combined with natural materials such as wood and wicker.