Lam merges several input files line-by-line. By changing the input or output line separators you can do fun things like interleaving lines from several files or making a simple template system. Lam can also pad or truncate input lines to given widths before merging. Lam is often distributed together with jot and rs (sometimes under the name bsd-utils).
The milter module for Python provides a python interface to Sendmail's libmilter that exploits all its features. Milters can run on the same machine as sendmail, or another machine. The milter can even run with a different operating system or processor than sendmail. Sendmail talks to the milter via a local or internet socket, and keeps the milter informed of events as it processes a mail connection. At any point, the milter can cut the conversation short by telling sendmail to ACCEPT, REJECT, or DISCARD the message. After receiving a complete message from sendmail, the milter can again REJECT or DISCARD it, but it can also ACCEPT it with changes to the headers or body.
Microbrew MicroSieve is designed to be a high speed spam filter for USENET news. Given the large amount of spam riding around in major USENET news systems, a spam filter has to be very fast and remain effective. It checks for duplicate and large articles, spambots, and binaries in non-binary groups. It also has path-based auto-accept and auto-reject, as well as cross-post limiting.