Top 12 Windows Spam Filters

POPFile is a powerful and flexible email classification POP and NNTP proxy that you can use to filter spam efficiently and categorize good mail automatically. Unfortunately, POPFile can grow a bit heavy on memory and cpu load if you have trained on lots of mail.

Spamfence is an effortless but highly effective spam (and virus) filtering service.
Its only real shortcoming is that Spamfence relies on forwarding and requires two different email accounts.

MailWasher Pro is a highly competent, usable, secure and time-saving spam filtering tool. Combining multiple approaches, MailWasher Pro achieves a solid spam detection rate and protects you from viruses to some extent, too. Still, MailWasher Pro could be a little bit more comfortable by integrating with email programs better.

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.

K9 is a wonderfully precise, easy to use and fast learning Bayesian spam filtering tool. It's a pity this gem only works with POP accounts and lacks remote administration.

Cactus Spam Filter is a really easy to use and pretty precise spam filter. It's a pity it only works with POP accounts and does not leverage its power for further pre-sorting the good mail.

Spam Bully is a great and efficient anti-spam tool. If you train it well, Spam Bully can rid your Inbox of unwanted emails almost completely.

Spamato filters POP and IMAP accounts for spam with the potential for high precision and plug-ins that make it easy to use in Outlook and Mozilla Thunderbird. Unfortunately, Spamato can be a bit overwhelming with its multitude of options and little help.

SpamExperts Desktop identifies and eliminates spam precisely and, thanks to its plugging right into the email transport, works without configuration with any email program and just about any email account.
It's a pity SpamExperts Desktop is a tad slow to process mail (though that can be countered by having it download mail periodically), and the process that lets you correct the filter's errors could be improved.

BullGuard Spamfilter plugs into Outlook, Outlook Express and Mozilla Thunderbird to filter spam quite effectively and without much effort on your part.

SpamBayes employs sophisticated analysis using Bayesian statistics to rid your email Inbox of junk mail precisely and in an effortless manner.

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.