http://www.washington.edu/pine/ Pine is a fast, easy to use, text-based mail client. Very useful on small systems where no GUI is required or available.
http://www.mutt.org/ Text-based and very fast. Supports both POP3 and IMAP. Doesn't have bugs, it has fleas...
http://www.mozilla.org/projects/thunderbird/ This is the e-mail part of Mozilla extracted.
http://kmail.kde.org/ The KDE mail agent
http://sylpheed.good-day.net/ GTK+ based, lightweight, and fast e-mail client
http://sylpheed-claws.sourceforge.net/ Extended version of the GTK+ based, lightweight, and fast e-mail client
Personal Information Management
This could have come under Diary, as well. Written using motif, this is a rather old yet surprisingly featureful PIM program. Requires X11 and lesstif or openmotif. (ThomasAdam, 2004)
Looks like Outlook's Calendar tool, but it's built on Mozilla's XUL technology. The current alpha version seems to work well when plugged into The Mozilla Browser, The Mozilla Thunderbird Email Client or The Mozilla Firefox Browser.