MSGTAG 1.4.5
MSGTAG provides an easy to use and convenient way to be notified when the recipient of an email you sent has opened it. Unfortunately, MSGTAG relies on HTML return receipts only.

Balsa 2.0.15
Balsa is a nice and easy to use Gnome email client with strong aspects (ease of use, message viewer and editor), but also weak spots (no global search, some text reflow problems).

1st Mail Checker 2.1.2
1st Mail Checker is a comfortable and powerful mail checker that can also be used safely by multiple users. More flexible filters and would be great, though, and 1st Mail Checker supports POP only.

New Mail Notifier 2.03
New Mail Notifier adds a very useful new mail notification window to Outlook.

RelayTest Pro 1.8
RelayTest Pro lets you test whether a (your) mail server is open for spammers to exploit for relaying their junk through it. It's best run from without your local network, though.

MailEnable Standard 1.71
MailEnable Standard is a simple, but robust and secure mail server.

Ximian Evolution 1.4.5
Ximian Evolution is the ambitious project of a fully featured groupware application for Linux, and a powerful, flexible and generally great email client, too.

MERCUR Mailserver 4.2
MERCUR Mailserver is a solid and capable choice and serves mail reliably. It lacks filtering options, though, and a strange limitation prevents you from accessing the same account with POP and IMAP.

Spamihilator 0.9.7.2
Spamihilator is a pretty, easy to use anti-spam tool that works with any email client and, thanks to Bayesian filters, has a good detection rate. Unfortunately, Spamihilator only supports POP.

Spam Interceptor
Spam Interceptor is an elegant, easy to use and flexible, but most of all effective spam filtering solution. Its combination of multiple strategies works great -- unfortunately for POP accounts only.

HideOE 1.1
HideOE provides fast and easy access to Outlook Express from the system tray and lets you check for mail or create new messages, too.

GNUMail.app 1.1.1
GNUMail.app is a slick email client with solid support for multiple accounts and powerful filters. Unfortunately, GNUMail.app doesn't support HTML and its plain text editing is limited, too.

VM 7.17
VM is a solid, tested and easy to use mail reader for Emacs. It can do all you expect from an email client and some more. VM is better used with POP and local mboxes, not IMAP, though.

WinAntiSPAM 1.18
WinAntiSPAM is a simple challenge-response spam fighting tool that works as a POP proxy. Unfortunately, WinAntiSPAM doesn't even use captchas for authentication.

SpamEnder
SpamEnder implements a simple concept (only mail from authorized senders is let through) in a simple and easy to use way. The challenging mechanism is too simple, though.

QuarantineMail 2.0
QuarantineMail removes practically all spam reliably by assuming everything from unknown senders is spam, but requires new senders to respond to a challenge.

Knowspam.net
Knowspam.net is a particularly easy to use spam filtering service. Its challenge-response way of banning all unauthorized senders is effective, but also introduces a number of problems.

geekmail.cc
geekmail.cc provides powerful, universally accessible (IMAP and web -based) email accounts with stellar spam protection and security. The web interface lacks some features, though.

(STEVEN) 3.7
(STEVEN) blocks a lot of spam by making sure only mail from authorized senders with a valid From: address gets through. It still suffers from challenge/response problems, though.

Mail PassView 1.13
Mail PassView automatically recovers not only your password but other account details as well and shows them in a clean window. It's a pity that Mail PassView doesn't yet work with all email clients.

MailMergeApp 1.1
MailMergeApp does exactly that: merging messages with the Mac OS X Address Book quite flexibly. Unfortunately, MailMergeApp lacks more advanced features and HTML editing.

Oddpost
Boasting a rich Web interface, superior Bayesian spam filters and support for RSS feeds, Oddpost makes a great email service. Unfortunately, it requires Windows IE, and lacks some features.

Pine 4.58
Pine is a flexible and easy to use command line email client that shines with IMAP accounts and in Unix environments, but is less usable on a PC or for POP access.

RssReader 1.0.29.0
RssReader is a straight forward RSS feed reader whose lack in features is, to some extent, compensated by its simplicity. At least the option to organize feeds in folders would be nice, though.

SpamWasher 1.2
SpamWasher is a friendly, easy to set up and use spam filter. If you don't use the whitelist extensively, the filters are overreaching, though, and likely to produce false positives.