Extensive list of plug-ins and add-ons for Apple Mail
Hawk Wings, an informative blog dedicated to Apple Mail, has a listing of many plug-ins and add-ons you might want to try. For example, did you know that there is an AppleScript called RCMail, that when attached to a rule in Apple Mail, sends commands written in an email to the shell for processing, logs the results and emails them back to you? Pretty good stuff so if you're a Mail.app user do check it out.
