I really think post cards are usually the most useless piece of mail besides "Free AOL" disks or Geiko car insurance offers.
Some jerk wants you to know that they had a lovely time on a beach as you froze your ass off working at a soul-sucking job in Neenah, Wis.
Then you get cards that basically make fun of themselves, and the whole idea of there being this far-off relaxing utopia. Particularly if they are postcards from Wisconsin, Indiana, Minnesota, or Detroit.
Awhile back, this card from a good friend made my day:
I had sent him one from Indiana saying that my girlfriend and I had driven through the state and eaten things.