Dragonfly Dream comes first, a most complete site with an excellent list of mail art calls and plenty more. It is getting a little complicated for little old ladies like me though.

In Boek861 you can register to get a regular bulletin and there you  can  also put a link to your site automatically.

In  Mail Art, the Forum  you can enter the details of your projects, which are immediately published, and see very clearly displayed the mail art calls published there by other artists, another great site.

Field Study International is now available on line in the Geelong Arts Alliance site.

Denis Charmot has created an interesting site which displays  not only his own Mail Art, a very good idea, but the artwork sent to him and also links to many mail artists sites and a list of projects.  

as well as Pati Bristow's site featuring her own Shopping Trolley Gallery - West

   And Jaimie Crofts's Mail Art Sonata is a very interesting experiment in which I am most happy to have participated. There are other exciting musical mail art projects here

Keith Bates has a cool site from where you can download the fonts he designs, some, like Mailart, are free

And the site of Bruno Capatti is also worth a visit

Bernd Reichert, has, in his very words, 'joined the 21st Century' and created a great site

If you understand French visit Artotem, the lively blog of Pascal Minart

Ed Giecek excels in hand-made Artist's Trading Cards, beautiful and plentiful

now that I am giving up the STG Michael Leigh has created a similar project, 
the Wheelie Bin Gallery 

Chantal Laurin  a new and enthusiastic mail artist, has a great site and a new project. Please give her your support.

there are plenty more, too good to miss but too many to list, unfortunately ...