MacBiff is a free, open source Mac OS X Biff program. It periodically checks your IMAP server for new email, and reports (from your Mac's menu bar) where in your folder hierarchy your new mail resides.
Version 1.1.15 adds/changes the following:
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Can't find what you're looking for? Try a search:
Also, if you have an older Mac, be sure to check out the "Classic" applications page for more options.
Finally, take a look at ALEMIA if you think you know that name of an application, but aren't quite sure.
Graham Orndorff has written a superb collection of articles on setting up email servers and secure email clients on Mac OS X.
Adam Engst has put together a comprehensive overview of email attachment formats that is invaluable for anyone who wants or needs to understand the complexities behind them.
These are applications that are newer and of potential interest, but which I haven't yet selected for permanent inclusion. Have a look, and let me know if you think they deserve to be part of the permanent collection!