To test it, I set it up to get my Google Mail, so I could compare its filter to Gmail's built-in functions. I also moved 50 messages from Gmail's "Spam" box to its inbox, to force SPAMFighter to work on a large load of real junk mail. Overall, SPAMFighter is almost--but not quite--as good as Gmail's filters, which are the best I've personally encountered. It pegged 36 out of the 50 'known spam' messages. Not bad, but it also marked several good messages as spam--and as valuable messages can be lost, false positives are often worse than receiving one ad for herbal Viagra. However, SPAMFighter allows you to unblock messages, and builds its whitelists and filters based on user response, so the program 'learns' as more people use it. Further, you can add your own entries to the black and white lists to customize the program.
Spamfighter Standard cannot be installed directly. The Pro version is installed and runs for 30 days. After that, the program auto-downgrades to the Standard version. This version is somewhat more limited than the Pro. The Standard version allows only 100 personal black/white list entries, places advertisements in your mail programs toolbar, and appends a footer to all your outgoing mail touting the virtues of Spamfighter. The last is probably the most irksome, as it turns your messages into vehicles for advertising--but it does mean getting a world-class anti-spam tool for free. Your call.