Tiny Prints is offering a free Valentine's Day card for today only (it expires at midnight PST). After liking them on Facebook, you'll be given a code for a free custom card. As with their previous promotions, shipping is free as long as you send it to your own home. If you try to mail it elsewhere, you'll be charged $0.44.
I plan on sending myself a card and then acting surprised when it arrives. The only danger is that I'll forget doing so and thus believe I've actually got a secret admirer.