PyWebMail is a Python library and POP3 gateway that enable working with WebMail servers like with POP3 or IMAP mail server. Currently Mail.Com, Yahoo! Mail, Gmail, Netscape.Net, HotMail and some minor servers support is finished.
gavamail is a POP3/IMAP server implemented in JAVA for reading your gmail e-mail with your favorite mail client. Currently only POP3 is implemented but I hope to implement IMAP too. Libraries provided by the g4j and libgmail projects are used.
mailgate is a set of Python scripts we used at my former company to transfer email a) from a POP3 account to our internal mail server, b) from a POP3 account to a mailbox file and c) from a mailbox file to a SMTP server.
JEEPERS (Jason's Extra Easy to Program Email Response System) uses a POP3 email account to monitor email for command phrases in subject lines. Default commands for remote file storeage and retrival. (Nothing to do with mailing lists or mass emailing.)