Top 13 Free Spam Filters for Windows

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.

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. You cannot remotely administer or teach Spamihilator, however.

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.

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

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 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.

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.

SpamRIP is a highly precise spam filter for any POP email account. A separate program, it lets you preview and delete junk before it reaches your default inbox. Unfortunately, SpamRIP does not work with IMAP accounts, is a tad clumsy to use and may crash now and then.

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