Lilja's Library posted a great article about all of the movie and tv appearances by King.
Did you catch Stephen King in Pet Sematary? Did you see his wife Tabitha in Knightriders? Did you see the ATM call him an asshole in Maximum Overdrive? You probably did but what you might not know is that King has done a lot of cameos in both his own movies and in others. If you’re up for it I’ll be happy to guide you so that after you’ve read this you’ll know exactly where to find them.
Check out the whole article here. Actually, read his whole site, it's the epitome of King fansites.