Camel's Eye is a client-independent cross-platform POP3 email filter written in Java. It is suitable for both spam detection and general mail filtering. Compared to most clients' built-in filtering systems, Camel's Eye has several advantages:

Its rules are more flexible and powerful.
They support regular expressions, scoring, multiple logical operators, variables, and more. Arbitrary message headers can be added, modified, or removed.
Messages can be analyzed without downloading them completely.
Various kinds of unwanted messages can thus be identified and deleted (or skipped) very quickly. This process is effective especially on slow connections and can save you megabytes of junk downloads (spam, newsletters, bloated attachments, etc.).
Automatic whitelisting.
Camel's Eye also acts as a proxy for outgoing messages. It automatically creates and maintains several whitelists (address, subject, domain, message ID) to protect mail from your contacts.
Rules can be shared with other users.
You can use "remote" rule sets created and maintained by somebody else, or publish your own rules. Camel's Eye automatically performs all required synchronization operations. This also works with dial-up connections.