You know I'm a fan of digital postcard services [1], so I did a little (discreet) dance when I found Postcard Express [2] ($4) for my iPhone.

You can take a picture at whatever fabulously exotic location you're at, and then you add your message and the email address of the recipient. It'll even geo-tag your photo/postcard for you.

The finished product looks so much like a traditional postcard, you won't miss buying the cards and making sure you have postage.