Sure, you can buy pumpkin-carving kits and pattern books -- but there are also a number of sites that offer great patterns for free. All you have to do is print them out and transfer them onto you pumpkin. Here are some of the pattern sites you can check out:
Family Crafts
Pumpkin Masters
The Pumpkin Lady
DLTK's Growing Together
Pumpkin Glow
DotCom Women