Top 25 Most Popular Mac OS X Mail Add-Ons

OMiC makes Mac OS X Mail handle winmail.dat inclusions as if it had invented them, making included files and rich formatting available just like other attachments.

Mail Attachments Iconizer makes all attachments show as space and time saving icons in Mac OS X Mail. Using the context menu, you can still preview the full attachment inline, and Mail Attachments Iconizer can be configured to turn into icons only certain attachment types or files exceeding a certain size.

MailTags lets you add tags, keywords, notes and due dates to emails in Mac OS X Mail and integrates them with search, rules, smart mailboxes, Calendar, Reminders and project-management software for near-perfect and semi-automatic custom email organization.

Mail2iCal and Mail2iCalToDo are highly useful AppleScript applications that let you turn any email from Mac OS X Mail into a calendar or to-do list item in iCal swiftly and with all the essential data (including URLs and attendants) entered. Mail2iCal could be a bit more comfortable, though, and possibly offer per-message options.

Mail Unread Menu displays the number of unread messages from Mac OS X Mail in the menu bar, next to the clock, instantly, and without any frills.
It's a pity you cannot open individual messages and Mail Unread Menu stutters displaying subject information.

Mail Act-On is a wonderful Mac OS X Mail plug-in that saves you time and makes your mail handling better by allowing you to assign keyboard shortcuts to Mail rule actions (and adds outgoing mail filters to boot).
You can set up shortcuts for labelling, moving or redirecting messages, for example, though creating a complex and functional Mail Act-On setup can be a bit complex itself.

Abee is a fantastic utility to import contacts from CSV files (to which every respectable email program or service exports) into the Mac OS X Mail Address Book with ease and elegance. You can map fields freely to accommodate for your every need and even save your mapping for later.

Email Archiver saves messages from Mac OS X Mail as PDF files including all layout, headers and attachments.

GPGMail enables Mac OS X Mail to use GnuPG message security. It lets you sign and encrypt, verify and decipher inline and OpenPGP/MIME messages comfortably and flexibly.

equinux Stationery Pack makes it easy to add hundreds of gorgeous stationery designs to Mac OS X Mail — and helps you find the perfect template for the occasion, too.