Something I have been involved with for a long time is mail art. I exchange with artists, writers, and have participated in groups who send and exchange print and text pieces. I love the idea of random art coming in the mail, and the freedom to create anything and just send it off. Unlike an online "send", it is a real, tangible object that you are putting out there to travel, and receiving such yourself is amazing. Here is an online submission link, though its nothing like the real thing.