Thunderbird - A free and very complete email program from the authors of Firefox. Very popular and fully customizable with add-ins. (XP/Vista 32/64bit)
SpamAware - Free add-in for Microsoft Outlook or Outlook Express. SpamAware uses SpamAssassin to score email and perform the selected actions.
MailWasher – Excellent email manager. Preview and or delete emails on the server (webmail is not supported). Includes white list and blacklist, visual and audio notifications, automatic deletion. Never download unwanted emails to your computer again. (XP/Vista)
Amic Email Back Up - Backup and restore email, settings, signatures, etc from Windows based email clients. (XP/Vista)
Trillian - Replacement for AIM, ICQ, MSN, and Yahoo messenger. One program that does them all. It's small, well written, and convenient. (XP/Vista)