Pegasus

http://www.pmail.com/ 

http://www.trimatrix.com/support/pegasus.html 

Outlook

http://www.microsoft.com/office/outlook/default.htm 

http://www.cc.ic.ac.uk/information/userdocs/win/gen/outcard.htm 

http://www.replymate.com/Download.htm  (An add-on for making more automated replies)

http://www.onenw.org/bin/page.cfm?pageid=36  (How to use rules/filters in Outlook)

http://www.caelo.com/  (An add-on for organizing emails in Outlook)

Outlook Express

http://www.microsoft.com/Windows/oe/ 

http://www.bigpond.com/Home/Support/Help/tutorials.asp 

Eudora

http://www.eudora.com/products/ 

http://www.silcon.com/howto/eudora/ 

Spam Filters

Inbox Protector http://www.inboxprotector.com/

Email Spam Blocker http://www.inboxprotector.com/

Spam Protector http://www.giantcompany.com/piOe.aspx

Other

http://www.fookes.com/  (Mailbag Assistant... a standalone program to help organize your email.)