TickerMyMail 3.0
With its stock ticker approach TickerMyMail is a very useful mail checker to use in the background.

Outlook Reminders by Email 2.0
Outlook Reminders by Email sends Outlook reminders to any email address without frills or much ado. It's a pity that you can't exert more fine-grained control over which alerts get emailed, though.

SendLater 1.59
SendLater makes it easy to schedule a message to be delivered at a later date, or even recurring flexibly in Outlook. There are no fields to customize individual messages, though.

Outlook-QuoteFix 0.90
Outlook-QuoteFix makes Outlook quote the original message correctly in plain text replies. Unfortunately, Outlook-QuoteFix doesn't allow you to reformat and rewrap text selectively, though.

Sendmail Enabler 1.1.3
Sendmail Enabler allows you to turn on and use the sendmail mail server built into Mac OS X with a few clicks to send mail everywhere independent of third-party SMTP servers.

MailCall 2.1
MailCall is a solid, easy to use and useful mail checker.

FreeBusy 1.13
FreeBusy is a handy tool that responds to email in Outlook whenever you're not ready to answer at once (because of a vacation or a meeting, for example). Its filtering intelligence is weak, though.

Sharp Mail Monitor 1.4
Sharp Mail Monitor offers a simple but nice way to announce new mail in a number of POP accounts. If you need more advanced features, though, Sharp Mail Monitor is not for you.

MailRelayer 1.4
MailRelayer provides an easy to use but powerful way to access all the messages stored in your email client remotely. It's almost like your own IMAP server, but you must use a supported email client.

Spam Sorter 1.5
Spam Sorter is nice anti-spam tool that comes with tons of filters. Unfortunately, its standards are too high, its rules too stiff to be precise and really useful.

MailObserver 0.6.1
MailObserver is a really simple but useful and unobtrusive new mail checker that informs you about incoming email in the status area of the menu. MailObserver lacks many features, though.

YahooPOPs! 0.5
YahooPOPs! provides reliable, flexible, easy to use and secure access to your Yahoo! Mail account in any email client. If it could also act as an IMAP interface, YahooPOPs! would be perfect.

PromaSoft AutoResponder 2.00
Highly sophisticated tool that makes your email life easier by automatically handling incoming mail in a number of ways, including automated replies.

AutoReply 2.1
AutoReply is an easy to use, but quite powerful autoresponder. Lacking advanced features needed for marketing purposes, AutoReply is perfect for holiday auto-replies, for example.

FilterMy.com
FilterMy.com is a simple and efficient spam and virus filtering service. Unfortunately, it only works with POP accounts and advanced users may miss some configuration options.

@Spam 1.0
@Spam is a spam filtering tool for Outlook with a few bright moments, but overall it is not precise enough and inefficient.

textSOAP for Windows 1.6
textSOAP for Windows is excellent for cleaning text, but it has its share of problems with quoting it for use in email replies.

MailSwitch 1.5.2
MailSwitch is a nice and useful utility if you want multiple people to use Mac OS X Mail, but don't want to go through the log-out/log-in procedure every time.

EmotiMail 1.0
EmotiMail makes it easy and a lot of fun to put animated emoticons in your emails. It comes with a large collection already, but lets you expand and organize your own smileys, too.

Schmaili 1.12
Schmaili makes it easy to enter funny animated smileys in emails you compose in Outlook Express, Outlook, Mozilla or Netscape.

SpamGunner 1.15
SpamGunner is a solid anti-spam tool for POP accounts. Unfortunately, SpamGunner's filters -- frequently updated and not useless -- don't achieve the accuracy of Bayesian or scoring filtering.

Plucky 1.0.3
Plucky is a simple newsreader that can not only display RSS feeds but flexibly extract new info from non-syndicated sites as well. It lacks tools to organize news and put them in context, though.

marlin 1.0
marlin is a nice email client with convincing security focus and a solid feature-set. It also comes with its own share of problems, though, among them poor IMAP support and a lack of proper quoting.

Mailinator
Mailinator lets you use any email address @mailinator.com and pick up the mail at their site. Since there's no connection to your real address, you sure won't get spam from using Mailinator addresses.

jetMailMonitor 6.1
jetMailMonitor is a clever and smart tool that will check multiple email accounts with a number of useful options including a blacklist of uwanted senders.