Pigspigot: User-Generated Greeting Cards
Pigspigot is a user-generated greeting card site that allows people to send custom digital creations to friends and family for free. It also has other social features, such as design rating and Twitter integration. For a minimal cost, users have the option to send their creations by snail mail as well. The only other site that comes remotely close is Moon Pig, but though it allows customization, card designs are not user-generated. From that point of view, Pigspigot definitely seems to have the upper hand.