maildirpop3d offers multiple POP mailboxes (with distinct passwords) controlled by a single shell account. It is a checkpassword replacement, and sits between qmail-popup and qmail-pop3d. If it sees a username like "joe-foo", and ~joe/Maildir-foo is a directory, it invokes qmail-pop3d (as joe) on ~joe/Maildir-foo. maildirpasswd (included) is then invoked by joe to set the password for the joe-foo mailbox. The password is then encrypted and stored.
Hep Message Server is software that transfers messages between different messaging systems. Eventually, it should transparently route messages between email, weblogs, and instant messaging. Currently Hep does one thing: fetches news feeds from Web sites and lets you view them in your email client.
Dovecot is an IMAP server whose major goals are security and extreme reliability. It uses index files to optimally store the mailbox state, which makes it very fast even with huge mailboxes. Indexes won't prevent external mailbox updates, so Dovecot is still fully compatible with standard Maildir and mbox formats. There's also a fully featured POP3 server included.
popwatcher is a daemon that maintains a list of hosts that are authorized to use your mailserver as an SMTP relay based on those hosts successfully authenticating to your POP mail server. The default configuration is designed to work with qpopper and exim, but can be used with other mailers and POP servers.