Mailvisa is a spam filter along the lines of Paul Graham's "A Plan for Spam". It classifies messages by comparing the words in them to known spam words. CPU usage, filtering speed, and filtering accuracy are comparable to other Bayesian filters. Mailvisa was originally intended as a platform for experimentation with different filtering parameters, but it has evolved into a useful spam classifier. In the author's own use, the amount of spam caught has surpassed 95% with no false positives so far.
libmime is a MIME parser in the same vein as Expat, the stream-oriented XML parser. As input is fed to the parser, events are generated which an application can catch by registering event handlers. Such events include the Unix From_ line, start of entity, end of entity, entity boundary, header, end of headers, and body. libmime supports MIME message editing through a delta mechanism. Edit contexts are instantiated and changes applied to specific contexts. Edit contexts can then be expressed in standard unified diff format which, when applied to the input source stream, will result in the new message.
qpsmtpd is a flexible smtpd daemon written in Perl. Apart from the core SMTP features, all functionality is implemented in small "extension plugins" using the easy-to-use object oriented plugin API. qpsmtpd was originally written as a drop-in qmail-smtpd replacement, but now it also includes an smtp forward and a postfix "backend".