I had heard about this but had never participated until recently when I joined Lenna Andrews Mail Art Swap. I chose for this one to make my own envelopes and make them artful. These are actually mailed individually to Lenna and she swaps them out and sends envelopes from other artists back to each participant in an art envelope of she designs herself.
Here are the 4 which I sent.
Garden of Dreams (front and back)
Gotta Love Mail Art (front and back)
Just Waiting to Hear (front and back)
Time For Mail Art (front and back)
It is a lot of fun and I'm very excited to see what mail art I get in return. I'll post them when they arrive!