whatsnewfm is a filter for the daily freshmeat newsletter. It shows only news items that you haven't seen before (so you don't have to read about every update of applications that you aren't interested in). You can also set up a list of "interesting" applications where you want to be informed of every update.
MIMEDefang is a flexible MIME email scanner designed to do all kinds of email processing, including anti-virus scanning, anti-spam scanning, replacing parts of messages with URLs, adding boilerplate disclaimers, and so on. It can alter or delete various parts of a MIME message according to a Perl-based policy filter. It can also bounce messages with unacceptable attachments and modify SMTP response codes on the fly. It works with the Sendmail 8.11 and newer "Milter" API, which makes it more flexible and efficient than procmail-based approaches.
Vipul's Razor is a distributed, collaborative, spam detection and filtering network that exploits the broadcast characteristic of spam distribution to limit its propagation. The primary focus of the system is to identify and disable an email spam before its injection and processing is complete. Razor establishes a distributed and constantly updating catalogue of spam in propagation. This catalogue is used by clients to filter out known spam.
parp is a powerful, extensible e-mail filter with sophisticated anti-spam capabilities. It's designed as a complete replacement for procmail, is MIME-aware, and acts as a filter, daemon, or on mailboxes. It uses the Open Relay Database as part of its spam-detection heuristics, and it has a DBM-format database of trusted e-mail addresses to ensure that false positives are kept to a minimum. It can perform duplicate removal by message ID, and has many other features.