Its hard to think of Gmail as anything other than an email service, but for a limited time this holiday season they are going to be in the business of mailing out postcards, by way of snail mail.
The holiday post card offer is going to be around for a limited time, and for a limited amount of people so if you want to send one you better not wait to long. As far as what you can choose from, they have a variety of six different designs, all of which can be personalized with a 255 character message. And yes, Google is even paying the postage, but that also means they are limiting your holiday post card sending experience to one per person.
I guess this is just about as close to a way to send some holiday cheer to that one person in your life that is not using email yet.