OS X Mail
is so good that you use it—and proficient enough to be worth a few good tools that make it even better.
Fit for your email needs, OS X Mail add-ons offer advanced labels, simplified interfaces, new message notifications, versatile filters, enhanced security, artful stationery and much more. Find 25 of the most popular Mac OS X Mail add-ons here.
(Updated October 2013)
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.
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.
Keaten House, Ltd
Email Archiver saves messages from Mac OS X Mail as PDF files including all layout, headers and attachments.
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.
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.
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.
IMAP-IDLE for Mail.app adds support for the IMAP IDLE command to Mac OS X Mail, which means new messages come in as soon as they arrive at the server without any manual or periodic mail checking. IMAP-IDLE works fine and unobtrusively, but support for both enabling IDLE for custom folders and disabling certain accounts would be nice.
Mac OS X Mail 3 and later does support IMAP-IDLE without the IMAP-IDLE add-on.