pgen is a little tool that uses m4(1) and some macros to help build and maintain a procmailrc file. It can make life much simpler than having to remember how procmail recipes work each time you subscribe to a new mailing list.
It won't help you maintain the worlds most complex procmailrc file, but for simple cases, it can make life a lot more convenient (and amenable to further postprocessing).
It's the configuration file for an advanced mail filter called procmail.