ECTool is a very popular program to play e-mail correspondence chess with. It has it's own e-mail interface, reads/writes .PGN format, is very customizable and is now freeware. To register it, simply do the following (taken from the ECTool web-site):

Once downloaded register it using the following details:

USER NAME : FREEWARE VERSION
PASSWORD : 108702118100

(Note : USER NAME IS CASE SENSITIVE)

• Latest Version: 7.01
• Homepage: http://www.ectool.nu
• Author: Andres Valverde andres@spainmail.com

One of the things I like about ECTool is that it is very customizable. You can create bitmaps to use for the squares to give things your own look. I have created a few sets and made them available for download. All pieces are the correct size to use with ECTool. Click on the thumb-nail example to download the appropriate set.