This is a difficult category, because the average user needs products that are easy to use. Frankly, most free spam filters are much too complex to set up and use. In fact, the best products in this class are shareware, and therefore not free. That said, the best free option for most users is MailWasher.
MailWasher is an email preview utility that allows you to check your email on your mail server before you download it to your PC. The advantage of this approach is that you can kill unwanted messages, including spam, viruses and large attachments before they get anywhere near your computer. MailWasher flags those messages which it assesses as questionable (i.e. may contain possible spam and viruses) for you to quickly check. It's a simple idea, but quite effective and one which average users find easy to understand.
At the Mailwasher site you can download a free or "Pro" commercial version. The former lacks quite a few of the features of its commercial cousin, the most limiting of which is the ability to access multiple accounts. However, the last free version of MailWasher (before it became a commercial product) is still floating around the web, and it will handle multiple accounts. It also lacks many of the advanced spam-detection features of the current commercial version, but is still very effective.
Alternatively, try xTerminator, which works in much the same way as MailWasher, and is totally free. I must say, though, that I prefer MailWasher's user interface.
If you use Outlook or Outlook Express for your email, then you have another choice, an excellent one actually, if you are willing to have a small advertisement appended to the end of your outbound email. SPAMfighter is a network-based spam-filtering system that uses the opinions of over two million users worldwide to help classify what is spam and what is not.
Like all network-based spam filters, it requires no training; it's ready to go the minute you install it. Each email is checked after it is received. If the email has been classified as spam by a lot of other users on the network, then it's deleted from your in-box and placed in a spam mail folder. It works like a charm and is a commanding choice for Outlook users. If you can't tolerate the ads in your email, you can have them removed for $19.95 a year.
Mailwasher Free (Feature reduced)
Website: http://www.mailwasher.net
Author: Nick Bolton
Version: 6.1.0
Date: January 9, 2008
Download File size: 7.20 MB
License: Feature limited Freeware
Operating systems supported: Windows 95, 98, Me, NT, 2000, XP, Vista
64 Bit Capable: No
Other languages supported:
Additional Software Required: No
Other relevant information:
Mailwasher (Last free Pro version)
Website: http://www.321download.com/LastFreeware/page2.html#Mailwasher
Author: Nick Bolton
Version: 1.33
Date: May 16, 2002
Download File size: 1.53 MB
License: Freeware
Operating systems supported: Windows 95, 98, Me, NT, 2000, XP
64 Bit Capable: No
Other languages supported:
Additional Software Required: No
Other relevant information:
xTerminator
Website: http://www.artplus.hr/adapps/eng/xterminator.htm
Author: Art Plus
Version: 5.0
Date: December 17, 2007
Download file size: 1.61 MB
License: Freeware
Operating systems supported: Windows 95, 98, Me, NT, 2000, XP, Vista
64 Bit Capable: No
Other languages supported:
Additional Software Required: No
Other relevant information: Can be used from USB key or memory card.
SPAMfighter
Website: http://www.spamfighter.com
Author: SPAMfighter
Version: 6.2.5
Date: December 17, 2007
Download file size: 11.8 MB
License: Freeware
Operating systems supported: Windows 95, 98, Me, NT, 2000, XP, Vista
64 Bit Capable: No
Other languages supported: English, German, Spanish, Chinese, French, Italian, Greek, Dutch, Swedish, Norwegian, Suomi, Japanese, Portuguese, Bulgarian, Russian and Danish.
Additional Software Required: Requires Microsoft Outlook or Outlook Express
Other relevant information: The free version will tag the bottom of each of your outgoing e-mails with a "Protected by SpamFighter" footer.