With Hippopost, you can either upload a photo or pick one of theirs; write a note and the address of the person you want to receive it; and then they’ll mail (and pay for) the hard copy postcard. Each postcard will have a supposedly small ad at the bottom, and you can choose which business is advertised.
This could a nice little exercise for English Language Learners — everybody likes to receive something in their mailbox, and I knew it would be an event for my students. Student’s names could be picked out of a hat to see who would write to who.
You can read about Hippopost in this TechCrunch post. They just released a Facebook application, but you can also do the same thing from the company’s main website without being a Facebook member.