http://expect.nist.gov/tkbiff/

tkbiff is a mail notification program.  Besides doing what it is supposed to, tkbiff is also a nice example of a polished Tcl/Tk application.  It's wonderfully configurable, well documented, and also serves as a Rosetta Stone for IMAP, POP, and file-based mail servers.  Tkbiff is also portable to all of UNIX, Win, and MacOS and there's a fair amount of code to make that happen just right.  (The Tcl language is portable but that doesn't mean that all programs are trivially portable!)