fidipidi, aside from being a portmanteau of serendipity and Pheidippides, is a Facebook application that allows you to create real-world greeting cards that get sent through the U.S. Postal Service for delivery to real human beings. Think of it like all those e-card services that were so popular about a decade ago, except that the card gets delivered to the recipients actual mailbox instead of their email inbox. Looks fun, and easy to use.
The first 15,000 people to sign up for fidipidi can use the code 1stCardFree to get their first card created and sent for free!
For a variety of silly reasons I don’t really use Facebook, let alone any Facebook apps, so I can’t provide a real testimonial here. If Facebook is your kind of thing, this might be a good app to try.