Email

Althea
An IMAP e-mail client for X-windows. It has support for using many servers at once and IMAP over SSL. (C++) [GNU/Linux]
http://althea.sourceforge.net/

Biabam
Mail attachments from the commandline. (Shell script) [GNU/Linux, Unix]
http://mmj.dk/biabam/

Fetchmail
A mail retrieval and forwarding utility intended to be used over on-demand TCP/IP links (such as SLIP or PPP connections). [GNU/lInux, Unix]
http://fetchmail.berlios.de/

Maildrop
A mail filter/local delivery agent with a perl-ish filtering language and maildir support. (C++) [GNU/Linux, Unix]
http://www.courier-mta.org/maildrop/

Mailfilter
A utility to get rid of unwanted spam mails, before downloading them from the POP3 mailbox. (C++) [GNU/Linux, Unix, Windows 9x/2000/NT]
http://mailfilter.sourceforge.net/

Mailsync
Synchronizes a collection of mailboxes, which may be on the local filesystem or on an IMAP server. (C++) [OS Independent]
http://mailsync.sourceforge.net/

WMpop
A WindowMaker dock app that monitors a POP3 mailbox to inform you when mail arrives. It can be configured via a GTK+ GUI. (C) [GNU/Linux]
http://jerome.sautret.free.fr/perso/wmpop/