odmrd is a server for the "On Demand Mail-Relay" protocol specified in RFC2465. It can be used as a standalone daemon or in inetd mode. It needs a MySQL server to store authentication and optional accounting information. It also requires a BSMTP-capable mailserver, such as Exim or postfix.
pb.WebMAUI (Web-Mail Administration User Interface) is a Web-based administration tool for managing complex mail domains, mail user accounts, server-based filtering including "out-of-office mail replies", and mail-related tasks. User quota managment is supported as well. Currently, the Courier mail suite is used, but support for other MTAs is planned.
perl-esmtpd is an antispam, antivirus, and authenticating SMTP daemon. It integrates with qmail, sendmail, stunnel, etc. It's simple Perl, so you can hack it to suit your needs. It supports RBLs, mx on from, bounce evil phrases, virus scanning, etc. It offers: RBL-sytle black-hole DNS checking when not relaying, verification of valid MX records for the MAIL FROM (when not relayin, the ability to use Stunnel for smtps connections and still do RBL checks etc., simple authentication (i.e. PLAIN or LOGIN) for offsite users, and the ability to reject email with certain key phrases. Many other useful features are included.
pine2mutt is a simple Perl script interesting to people who switch from using pine to mutt. It converts pine's .addressbook into mutt-readable aliases, enables pine-like sent-mail-folder handling and provides pine-like keybindings by sourcing the file "Pine.rc" which comes with mutt.
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.
purepost is an interface between Courier-MTA and Sophie for virus scanning. You can have it check mail for all your users, or just a few. Using courier's dot-courier functionality, this script intercepts an email before it hits the mailbox, creates a temporary file, tells sophie to scan that temporary file for viruses, and if it finds a virus, denies delivery and notifies the sender (otherwise it allows the message to be delivered).