Here is a listing and description of Tcl-based mail notification tools. This is not a comprehensive list. Please don't add last night's hack. Indeed, many people have written biffs - it's so trivial to write a simple one. But few are truly professional, robust, flexible, etc. Here are several that appear to be worthwhile.
[http://expect.nist.gov/tkbiff]
GUI: Shows message headers. Click to show messages.
User-definable action upon new mail: Message-dependent.
OS: Windows, UNIX, MacOS
Access: [IMAP], POP, APOP, SSL, UNIX mbox and maildir. (included with tkbiff - see above)
UI: "You have mail from ..." on stdout.
User-definable action upon new mail: Message-dependent.
OS: UNIX
Access: IMAP, POP, APOP, SSL, UNIX mbox and maildir. [http://www.linux-kheops.com/pub/xbiff2/xbiff2GB.html]
GUI: Shows icon for no mail or new mail.
User-defineable action upon new mail: Message-independent.
OS: Windows, UNIX
Access: IMAP, POP, UNIX mbox [http://www.gnt.net/%7En5ial/Tcl]
GUI: shows mailbox names containing new mail.
User-definable action upon new mail: No.
OS: UNIX
Access: UNIX mbox
Requires: TclX [http://www-personal.umich.edu/%7Emarkcrim/tkgoodstuff/]
GUI: Shows icon or count of messages in multiple mailboxes
Click to show messages.
User-definable action upon new mail: No.
OS: UNIX
Access: IMAP, UNIX mbox and MH.
Status: Development ceased as of Oct '97. [ftp://ftp.procplace.com/pub/tcl/sorted/packages-7.6/x11/tkpostage-1.3b/tkpostage-1.3b.tar.gz]
GUI: shows count of messages. Click to show message headers.
User-definable action upon new mail: No.
OS: UNIX
Access: UNIX mbox
Status: appears orphaned, last release: 1.3b, Dec '95