A wide variety of materials can be used.  Thickness can range from 1/16 thickness to 1/4, depending on where the signage will be used.  Coroplast is commonly used for yard signs.  Sintra can be as thin as 1 mil. to bend around display units for trade shows, or as thick as 1/4 for outdoor signage.  It comes in a variety of colors and can also have die cut shapes.  Acrylic is common for lighted signs and professional office signs.