Mozilla Thunderbird is an open source e-mail suite. It comes with a junk-mail classification capabilities using the Bayesian Filtering technique. It requires you to first train the program by showing it a bunch of mail that is junk, and a bunch of mail that is not. Then, you can let it autoclassify new mail for you, while you continue to correct it as it make mistakes. After a few iterations of training, the program can achieve amazing accuracy in classifying your good and junk e-mails
If you prefer to stick with an e-mail client that you have been using for years, you may want to give POPfile a try. POPFile is maintained as an open standard by a team of developers and users.
POPFile is a proxy which sits between your e-mail client and your e-mail server. Commands generated by your e-mail client are passed through POPFile to the e-mail server. As messages are retrieved, POPFile reads the incoming messages, tries to classify them to the categories you have defined and then passes them on to your e-mail client.
For Outlook users, you can also try SpamBayes as it comes with a plug-in for Microsoft Outlook which tightly integrates classification and training into the Outlook interface. SpamBayes is also maintained by a group of open source programmers. The team boasts of having new and refined algorithms in their program that can offer greater accuracy in determining spam.
SpamSieve (For Mac OS X User)
For the Mac OS X user, the built-in e-mail client Mail.app already comes with good spam-filtering mechanism (see The Fight Against Spam article for more information). However, if you want to try out something else, Spam Sieve is a good candidate that many Apple fan sites have touted.
Other Software (including commercial products)
Many of the modern e-mail programs today already come with spam filtering mechanism. You may want to check the program documentation/helpfile or check with your software vendor on how you can make use of their spam control features.
For a list of additional software options, you can also check out the following links:
CNET Download Center under the Spam Filters Section
Stroud's Consummate Winsock Apps List under the Mail client section