GMailWatch is a GNOME Panel Applet that displays a summary of incoming mail (sender/subject) rather than just an icon. It's similar to the old xmailwatcher utility (and uses some of the same code), and is also based upon mailcheck (of gnome-core). The user can change the size of the panel applet.
Gwatch is mail spool monitor similar to the watcher feature of tkrat. Gwatch monitors a mail spool, and when new mail arrives, it displays a window, containing the from, and subject fields of the new email. It is written in C, with the GTK+ toolkit, and developed on Linux. It is known to compile and work on FreeBSD, and should compile and run on most *nix platforms (see the Build Notes in the documentation). Note: Gwatch is still in the ALPHA stage. It may break at any time.
The Helpdesk::Mail System is a Perl application intended to assist Helpdesks and Support Centers in organizing their email communication with their customers. Especially if there is one email address and several people who reply to those emails and answer questions, Helpdesk::Mail can come in handy. It is not just another webmail application; one can assign a "user-id" for an e-mail (e.g. an account number) to track how often one user asks for help. It also has an additional "Knowledge Base" feature: if your customers ask the same questions over and over again, you can save the answer in the Knowledge Base and prefill a reply with this answer by simply typing in some keywords and click on a link. There's also a public interface to the Knowledge Base.