Creddon - Tweet for a cause and a sponsor will donate $1 to charity
AI Postcard Generator - Generate postcards with image and text based on your input
Send Mail Get Mail - A postcard-sending website.
Wishful Roots - Send cards that grow your wish into living plants!
Postacard - Text a photo to send it as a postcard for $3.
Felt for iPhone - Handwritten cards for the modern world