At Postcrossing you swap postcards with other persons, selected randomly. By each postcard that you send you receive another back, by a different user.
There are other websites with the same purpose but Postcrossing has the larger number of users (+700k) and countries (+200).
I registered myself in 2008 and after some years I have restared postcards again. You can check the postcards that I have received and sent here: Postcrossing:oort.

Here are shown three examples of the received postcards: one from Shangai (China), another from Auckland (New Zeland) and another from Split (Croatia).