plopmail is a pure python MDA replacing procmail or maildrop
The filter scripts are written in python, with a simple syntax to be able to acces any mail header and attribute.
It automatically creates maildirs and notify the user by mail when it does. It only works with courier-imap for the moment, but support for other imap servers is planned.
It also comes with plopmonitor, a gamin compatible daemon to monitor maildirs and execute script when needed (useful to reclassify mails).