Fwsecvpop3d has been designed to be secure, fast and reliable pop3 server. It's easy to configure, supports many authentication features, virtual users (per-domain auth, so the same user can exist in a lot of domains) and is thread based (scalability).
PoCoMy: Postfix + Courier + Mysql :: Webfrontend for administration. Features: Virtual domains, forwarding, aliases, sender aliases, multi user, multi language, privileges system, delegation of rights per domain, ...
PinoyPHP Mail is a PHP Webmail E-mail Client which create virtual accounts usable by many users. Requires MySQL, and ONE POP3 account. Added: New Registration, Sent Items, Improved Mail Client. See demo at: http://mail.pinoyphp.info (you may register)
After the presentation on CeBIT 2004 fair finally available: SpamStop works as a Proxy between your Pop3-Client and the -Provider, it uses a Support Vector Machine to classfy incoming mails and automatically learns classifiers from samples of your mails.
phpMEQPopAdmin is a small collection of PHP4-Scripts to administrate the mysql-tables for your virtual mailserver (http://www.netd.co.za/mysql-mail/). The admin of each domain can logon and create pop-mailboxes, forwarders and catchall-accounts.