Pumpkin carving stencils are available at many sites, such as DLTK Holidays, Extreme Pumpkins, and The Pumpkin Wizard. The stencils are .pdf files and include instructions for carving.
DLTK Holidays provides an extensive array of patterns, from aliens to zombie hands. Each pattern includes a downloadable .pdf file and a photo of the finished product. The patterns range from easy with few cuts to more complicated patterns with difficult cuts.
Extreme Pumpkins is a free downloadable book of patterns. The author claims to have the grossest, silliest, and most disturbed patterns available. The book includes 48 patterns, two sets of fonts, carving tips, and photos to help carve the pumpkin.
The Pumpkin Wizard has over 100 free patterns and over 1100 patterns for purchase. The website specializes in toned patterns, meaning the carver has to peel the skin and some of the flesh off the pumpkin. This adds greater depth and realism to the final product.