Okay, first of all, don't buy vinyl shower curtains! They off-gas the plasticizers they're made of and these VOC's are endocrine disruptors, which means they mimic natural hormones like estrogen and may interfere with a developing fetus. VOC's are eye-irritants, central nervous system damagers and possibly cause cancer. You can buy really tightly woven fabric curtains instead (I bought mine at Sears) or nylon ones which don't off-gas. They last indefinitely and you can wash them in your washing machine. But if you already have vinyl shower curtains, here are some ideas from Trash Talk on how to reuse them:
"Vinyl shower curtains reused make good drop cloths for painting projects. In the garden they can be used as row covers for tender spring plants (when a cold night threatens). Like costly landscape fabric use curtains to control weeds in garden beds. Cut an X at each plant location, plant, then close the X around the plant stem. Cover the entire sheet with rocks, gravel or bark mulch."
They also suggest using them as ground covers under your tent when camping, using them as liners in the trunk of your car, or cutting into big rectangles and securing at the shoulders with clothespegs as craft smocks.