I also participate in a number of art swaps. I have swapped post cards for quite some time with two groups. One is The International Post Card Swap hosted by Shannon Ganshorn from http://musingsonrealities.blogspot.com.au/ and the other I hope to continue is hosted by Hanna from http://www.ihanna.nu/.
These are lots of fun and the anticipation waiting for returned mail is exciting.
http://willowing.ning.com/ I really enjoy the communication and getting that lovely happy mail from overseas.