Microsoft
Outlook - The de facto standard in corporate
groupware messaging, especially when corporations utilize
Microsoft Exchange e-mail server.
Microsoft
Outlook Express - The free e-mail software companion
to Internet Explorer. It is the most widely used e-mail
program, but version 6.0 hasn't seen a substantial update
in several years.
Mozilla
Thunderbird - The version 1 release of Mozilla's
open-source e-mail client alternative to Microsoft Outlook,
with the ability to add functionality through software extensions
built by the development community.