SpamSieve

The best spam filter I've used.

Requires Mac OS X 10.2 or later (10.3.5 or later recommended)
Works with:
Apple Mail
Emailer
Entourage (9.0.1 through 2004)
Eudora 5.2 or later (Sponsored or Paid)
GyazMail
Mailsmith (bundled with Mailsmith 2.1)
Outlook Express 5
PowerMail (optional bundle with PowerMail 5)

Free Trial: fully-featured (30 days or 20 launches)

SpamSieve gives you back your inbox by bringing powerful Bayesian spam filtering to popular e-mail clients. It learns what your spam looks like, so it can block nearly all of it. It looks at your address book and learns what your good messages look like, so it won’t confuse them with spam. Other spam filters get worse over time as spammers adapt to their rules; SpamSieve actually gets better over time as you train it with more messages. SpamSieve doesn’t delete any messages—it only marks them in your e-mail client—so you’ll never lose any mail. SpamSieve works with any number of mail accounts, of whatever types are supported by your e-mail software (e.g. POP, IMAP, Hotmail, AOL).

Powerful Bayesian spam filtering results in high accuracy and almost no false positives. It adapts to the mail that you receive to get even better with time. Some other e-mail clients include Bayesian filters, but SpamSieve is more accurate.

Integrates with your e-mail client for a superior user experience.

Integrates with the Mac OS X Address Book (and also Eudora and Entourage’s address books) so that messages from friends and colleagues are never marked as spam.

Automatically maintains a blocklist so that it can instantly adapt to spam messages sent from particular addresses, and catch 100% of them.
• Automatically maintains a whitelist to guarantee that messages from particular senders or mailing lists are never marked as spam, without cluttering your address book with these addresses.

You can customize the whitelist and blocklist, adding sophisticated rules that match various message headers, or the message body. The rules can match text in a variety of ways, including using regular expressions.

Can use the Habeas Whitelist, which indicates messages that are not spam, as well as the “ADV” subject tag indicating that a message is spam.

Many spammers encode the contents of their messages so that filters cannot see the incriminating words they contain. SpamSieve can decode and look inside these messages. Optionally it can mark them all as spam, on the theory that legitimate senders do not try to obscure their messages.

SpamSieve keeps track of how accurate it is, how many good and spam messages you receive, and how these numbers change over time.

Turn off new-mail notification in your e-mail client, and let SpamSieve notify you only when you receive non-spam messages.

The corpus window and log let you see how each spam message was caught.

http://c-command.com/spamsieve/