I’ve definitely enjoyed using our photos as postcards, but even though I can use our free photos, postage will still cost (once I use up my free photo stamps). However, I recently discovered Hippopost, which allows you to upload a picture of your choice to be used as a postcard (you can also add a message). Hippopost will then print the postcard and the postage is covered by advertising (mailed to the US and Canada only–I’m guessing you could send them if you are located internationally, but would have to send them to a US or Canadian address).
They are able to do this for free by placing a small advertisement on the postcard. So long as you don’t mind sending a postcard that has an advertisement on it, this is a great freebie! I just ordered a postcard, so I’ll update once we receive it. If you’re on a trip and want to send a postcard–all you need is an internet connection and a digital photo (you can actually select one of their images and just add your message, too)!