I'm very fond of SpamAssassin. I set it up for myself, then my "provider" (really a friend's box) started using it at a system level. It's trivial to set up, makes use of existing tools and lists to identify spam, and is easy to customize. It doesn't do hard and fast decisions about what spam is or isn't, but let's you set you mess with the weightings and add conditions.
In my opinion, the best way to use a spam filter like SA is to allow it to tag spammish incoming messages, then make it really easy for individual users to filter out tagged spam using their mailer of choice (handing out instructions and example filters, for instance). SA has many configuration options to allow users to make small config files to set their spam tolerance level (some messages are obviously spam, but some aren't) or whitelist as they choose. One of the unexpected bonus benefits of SA is the hillarious comentary it adds to typical messages. Also, you can have contests for the highest ranked spam messages. The highest I've ever got was 51. Lauren --- Send mail for the `bblisa' mailing list to `[EMAIL PROTECTED]'. Mail administrative requests to `[EMAIL PROTECTED]'.
