Actif Mail
A nice email program comes from Actifsoft in four languages: English, Español, Français and Deutsch, offering RTF and HTMail, filtering, an interface to PGP and more.
Agent
Newsreader features for mail. Mailreader features for news. Fort'e Agent does that really fruitful symbiosis in a simple, realiable and highly usable manner.
AK-Mail
Great email client, reliable, usable and useful. This comes at no surprise as AK-Mail really reminds me of PMMail. PGP-support and HTMail (sigh).
Win95+/NT, both German and English.
The Bat
The Bat comes with many powerful, useful and useable features (e.g. multi-account, multi-user capabilities, security, and international language support) that make it one of the email clients that really stand out.
Becky! Internet Mail
A neat Email client from Japan with a powerful editor (maybe the single most important thing of any Email software).
BravoMAIL
BravoMAIL works like a camcorder, recording what you say, paint &
write. The receiver can watch the video. They call that "NETural
communication", I call it "necessary?".
CALYPSO
CALYPSO's strength is its multi-user and multiple-account orientation. There's also this nice (?) feature called "Blind Send" where every recipient of a mailing list sees only her name in the to: field.
Canine Mail
With Canine, you can access multiple POP3 and Hotmail accounts. Trying to stay lean it tries to provide only features that are really needed and supports non-textual content via IE.
cc:Mail
Lotus/IBM's strong email client/server now also with Internet functionality. Robust, usable,
used, and now -- some may say fortunately -- phasing out.
DTS Mail
Email client with a lot of useful features and the ability to access up to six pop servers.
email97
The email97 email client is as nice as its website. Feature-rich
and still simple (yes!) it is more useful, though.
Eudora 
Qualcomm's feature-rich, yet easy to use email client, in fact one of the best
email programs on the market, is available in a light, pay-for, or ad-supported
version.
FoxMail
The maxim of this tiny email client is: "small, fast, easy"; yet it is powerful enough even for "power" users.
JS Office
JenniSoft's Office package includes an integrated email client that supports an unlimited amount of email accounts and PGP encryption.
LingoMAIL
LingoMAIL can be used as a "conventional" email client, but its international language support makes it shine: >30 languages in one message. A viewer (sigh) allows others with Win32 machines to read it, too.
Mail Explorer
Read and write mail with the Windows Explorer. Not that the original explorer was made for this, but it works.
MailCat
MailCat has the usual (complete) feature-set which is extended with possible mail retrieval from Compuserve and via SMTP. The latter is an interesting mess of things. I think a mess should not evoke negative feelings.
Neoplanet
Neoplanet will change the face of your email. A full featured yet simple to use skinnable email client that comes with Web browser, too.
Netscape Messenger
With the advent of Communicator Netscape's Email client can really be called "full-featured" with its support for most of the current standards (including S/MIME) and platforms.
Microsoft Outlook
Part of the Microsoft Office application family, the personal information and email manager, Outlook, is how Microsoft tries to set the standards for email.
A very complex and powerful email client.
Microsoft Outlook Express
The current standard for email clients as they say. It's got a pretty (inter)face and it works pretty well. The filtering could be improved, though.
PC-Pine
PC-Pine brings the familiar Pine power and flexibility of the Unix text-mode email client to DOS and Windows. PC-Pine 4 is only available for Win32, though.
Pegasus Mail
Pegasus Mail is one of the most powerful email clients and it is a free download.
PMMail
One of the things that put OS/2 ahead of Windows for a long time is PMMail, an extremely usable email client with a clean interface. Windows 95/NT (3.51+)
PostMe
PostMe is a nice, easy to use, and still quite powerful program -- and it's free. The catch: it's in German.
QuickMail Pro Client
The client for QuickMail Pro comes with an easy-to-use and intuitive interface, usable filtering and remote access.
Route1 Mail
Route1 Mail is a powerful free email client. It is said to be the first with push technology. Oh, yes, this here now is the first
pull web page. Revolution!
Shark!mail 4.0
Impressive email client that works with just about everything (POP3, Lotus Notes, Microsoft Exchange,...) and has a strong filtering support as well as a strong editor, the three things we really want to see in an email client.
StarOffice
With StarOffice Sun has a free office suite that is ready to match with MS Office. The suite includes a capable and versatile email client.
TransSoft Mail Control
Browsing the features of this packed email client I saw things like "full-blown FTP client" or "Mini WWW-browser". Great, but: do we need that?