This one is for the Mac users in the audience. The Apple Mail Plugins and Tools
page offers quick links to almost every known plugin for Mail.app. When I stumbled on it the other day, I was amazed at the range of functionality people have added to Mail, especially given the undocumented plugin API. Some of my personal faves, like the all-important GPGMail
are featured, as well as some nifty autoresponders and templaters to handle form letters and mailing lists. A word of caution, though: there seems to be a "Hotel California" check-in policy for the list. There are some jewels, but there are also some broken links and apps that don't seem to have been updated since Jaguar.
Also, don't forget to scroll down to the bottom of the page, where the authors have set up an open "wanted" list for Mail functionality. Again, some of the gripes are outdated and have been fixed by Apple, but it still interesting to see all of the suggestions in one place. Hopefully, the Mail team is watching.