Unix

archivemail
Python tool for detecting and compressing older e-mail messages.
http://archivemail.sourceforge.net/

BSD Vacation V2
RFP-compliant vacation mailer, designed to replace the standard Sendmail vacation program.
http://www.weird.com/~woods/projects/BSD-vacation-v2.html

Fetchmail
Fetchmail is a free, full-featured, robust, well-documented remote-mail retrieval and forwarding utility intended to be used over on-demand TCP/IP links (such as SLIP or PPP connections).
http://fetchmail.berlios.de/

The Fetchmail FAQ
Frequently asked questions about fetchmail.
http://catb.org/~esr/fetchmail/fetchmail-FAQ.html

maildrop - mail delivery agent with filtering
Maildrop reads a mail message from standard input, then delivers the message to your mailbox. maildrop will optionally read instructions from a file, which describes how to filter incoming mail. Instructions can be provided having mail delivered to alternate mailboxes, or forwarded using a structured filtering language.
http://www.courier-mta.org/maildrop/

Mailfilt, by Mike Fisk (Unix/Perl)
Mailfilt is a procmail-like Perl application for filtering your mail. (Last updated sometime in 1995 ...)
http://infohost.nmt.edu/~mfisk/style.cgi?unixtools/mailfilt.html

Muttils
Provides Python a list of tools to facilitate the use of mail clients.
http://www.blacktrash.org/hg/muttils/

Pygmy
A GNOME mail client written in Python. Pygmy supports attachments and MIME messages, both in the message composer and in the message viewer.
http://pygmy.sourceforge.net

Vacation Auto-Reply (Unix)
Simple instructions for one variant of the vacation(1) program on Unix. (This doesn't appear to be the most standard version, but the general instructions look relatively ok.)
http://www.emailman.com/vacation/