The Simple Postcard - Text a photo to mail it as a postcard
Frysbe - Frysbe: Make global friends at random and the fling app comes up with features to send messages to strangers from every corner of the world to expand your social circle.
Felt for iPhone - Handwritten cards for the modern world
Unbordered - Unbordered app comes up with features to help you in finding the perfect language practice and exchange partners so you can practice foreign languages right through your mobile phone.
Send Mail Get Mail - A postcard-sending website.
PenPal World - Connect with people all over the world