pop3download is a script that downloads e-mail from a POP3 or IMAP mailbox. It was originally created to work around certain fetchmail bugs, and now works as a very limited and simple alternative to it. It can keep files on the server, force CRLF line endings, poll at an interval, and pass mail through an MDA for filtering.
Trojita is a Qt IMAP email client. It is a pure Qt4 application with no additional dependencies. It has a robust IMAP core implemented using Qt's Model-View framework. Standards compliance is a design goal. It has support for bandwidth-saving mode aimed at mobile users with expensive connections. In addition to the usual SSL/TLS connections, the server can be accessed via SSH. It has on-demand body part loading, offline IMAP support, and safe dealing with HTML mail (actually more robust than Thunderbird's).
S22.Imap is an easy-to-use and well-documented .NET library component for communicating with and receiving electronic mail from an Internet Message Access Protocol (IMAP) servers. It supports IMAP IDLE notifications and IMAP over SSL, allows selective fetching of parts of mail messages, is inherently thread-safe, and is well documented with lots of example code.
DavMail is a POP/IMAP/SMTP/Caldav/Carddav/LDAP exchange gateway allowing users to use any email/calendar client (e.g. Thunderbird with Lightning or Apple iCal) with an Exchange server, even from the Internet or behind a firewall through Outlook Web Access. DavMail now includes an LDAP gateway to Exchange global address book and user personal contacts to allow recipient address completion in the mail compose window, and full calendar support with attendees free/busy display.