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