AfterLogic WebMail Lite PHP is a free AJAX webmail application. Users can receive, view, manage, compose, and send email through web interface (IMAP and SMTP supported). Source code included.
PHP Mail Central (phpMC) is a web based multiuser email client written in PHP. It uses a mysql backend to store all collected emails (from POP3 or IMAP4 accounts) in a local database.
TiM is an email client written in PHP with a MySQL backend for storing mail. It can currently retrieve messages from POP3 accounts. Files released on project homepage.