Although we don't send out holiday cards, Red Stamp (a free app!) makes it a bit too easy to create cute postcards on your iPhone. Send a virtual one for free, or for under $2 they will mail an actual paper postcard for you. The simple fact that they use iPhone's address book makes me giddy. So user-friendly and the designs are fantastic. If we were to send out a card, this would likely be it.
Hope everyone is having a very merry holiday!