Websites with call lists:
kiyotei's den - kiyotei@beer.com
Mailart.org - contact form
Susannia - planet@susannia.de
Notta Caflisch - info@notta.ch
Consulate of Culture - mailart@leukipoli.gr
ASN mail art calls
Concrete Postcard
Planet Dada - online contact form
Art Correo - mailart@vorticeargentina.com.ar
Francois Frisch - frfrisch@hotmail.com
Luiza's Art Cafe
Museum of the Essential
Invasion Records - invasion@hispeed.ch
Francoise - fa-pictures@flashmail.com
A. Sleep - online form
Eddie Willson
Dragonfly's Dreambook
Zybooks - zyarts2000@yahoo.co.uk
Dystatic - Form on bottom of page
Viva Las Vegastamps - online contact form   
Mail Art Mekka Minden - Online Form

Mailing Lists:
Mailartist.com List
The Carving Consortium
IUOMA Mailing List
Blast
art-themagazine.com
Fluxlist

Directories:
DMOZ
Blogshares

Addresses:
Frappr Mail Art Group NEW!

Blogs:
IUOMA - TAM
Mick Boyle
qbdp
kiyotei's blog
Frips Mailart Blog
JCSynthetics UK
A.1. Mail Art Archive
5 Million Copies
84 Rooms
ed'zZ Blog

Message Boards:
Merlin's Mailart List
IUOMA Forum
One World
Live Journal
Chris Horst's Board
Craig's List
Hit Parade Mailart Forum

Groups/Rings:
Paint-n-thingsATCs
Artistamp News
Google Mailart Group
Mail-Art Network
ATC Exchange
Nervousness.org
Mail Art WebRing
ATC WebRing

Newsgroups:
Alt.art
Alt.zines
Alt.art.scene
Misc.creativity
Rec.arts.fine
Rec.crafts.rubberstamps

Ezines:
Musea
Art e-zine
Mondo QT
Twopeanuts.com
Tabloid Trash
Shouting at the Postman
Zimmerzine
ZineStreet

More Blogs
Frog & Toad's Wild Ride
MailXart
Anabasis/Xtant
Sghinismo nel mondo
RF Cote
Mailboxes
Exile on Main Street
Jassy Lupa
Crescent House

Mailart Documentation:
Doc's Page