http://www.sito.org/synergy/
Synergy is SITO's name for "collaborative art" projects. To participate in any of the currently active projects, you'll need a SITO ID, a three-lettercode used to identify yourself all over SITO.

http://www.starbuckseverywhere.net/
I have been on a personal mission to visit every Starbucks in the world

http://mailartprojects.blogspot.com/
mail art projects site

http://mailmeart.com/going-postal/
Going postal with the world's best illustrators and designers

http://www.keithbates.co.uk/
What is a Mail Art? Mail Art can be as simple as having lots of artist penfriends and exchanging art by post. It's also a bit deeper than that.

http://www.crosses.net/
Mail art networking site

http://www.art.net/~kiyotei/mailart/
Kiyotei's mail art site

http://www.art.net/~kiyotei/mailart/sbucks.html
Kiyotei's coffee card mail art project

http://www.squidoo.com/twitter-mail-art
Squidoo twitter mail art site

http://imagmailart.tripod.com/
International Mail art site