There is also a Postcrossing Forum to send unoffical cards in the course of tags, trades, RRs or others. You can find it here: http://forum.postcrossing.com/
Trough this forum I have got to know some very nice people from all over the world. Postcrossing is also a great way to find penpals.
Above I posted a map of official and unofficial received cards during my year and three months at Postcrossing.