Cartolina is an app that I noticed over on the great Drawn! illustration blog. While you might not want to get it yourself (it's the thought that counts with greeting cards, and I think that, with both cards and letters, the real thing is still better), you can't disagree that the art of this one is pretty incredible. Cartolina is originally a stationery company, printing cards and letterheads with elaborate and elegant designs on them, but they've also released an iPhone app. For US$2.99, you get access to nine different artistic templates, which can then be made into beautiful email or text messages. It's not a big deal (and it's no substitute for actually writing someone a letter -- something I already don't do enough of), but for classing up the standard congratulatory or simply nice email or text message, it'll work wonders.
The app also has a built-in calendar with reminders of major events, and the app promises even more designs and features coming soon. Again, if you already have a schedule and a plan for sending greeting cards, the app probably won't interest you that much. But it's always nice to send someone a reminder that you're thinking of them, and having Cartolina around might help you do just that.