PGMail is an open source plugin for Apple Mail. It brings the functionality to sign, verify, encrypt and decrypt mails using the OpenPGP standard.
Please have a look at the getting started and FAQ section. It would be nice to tell Apple to support bundles for Mail.app officially.

This is SaneBox, an impressive auto-filtering application for Gmail. I tried it before Priority Inbox was released, and then disabled it to try on Priority Inbox. Now I use both, but SaneBox is a more effective app for managing email.

Priority Inbox provides a visual hierarchy of your inbox, which is nice, but not a game changer. You still have to be an Inbox Zero Jedi or set up a bunch of filters and labels to achieve email zen. SaneBox to the rescue. The app learns from your email behavior and then automatically filters less important email out of your Inbox and into labels that SaneBox creates for you. This is a game changer. Before SaneBox, I would either manually process every email (except for spam) or set up countless filters and labels that would never be complete. After SaneBox, I estimate 70% of email that used to hit my inbox now go into the new SaneBox labels thanks to the SaneBox filters. I scan the mail in those labels a couple times a day, but don’t open every one. In fact, I don’t open most of them. They’re ignored and eventually deleted. It’s awesome.

Then I discovered the @SaneTomorrow and @SaneNextWeek labels, and my opinion of SaneBox went from great to epic. When you move any emails into the @SaneTomorrow label, the email disappears from your Inbox but then reappears the next day. The same logic applies to the @SaneNextWeek label. So now I can save important emails for later, but still remove them from my Inbox. I guess you could achieve this by starring these emails, but that requires the user to address the starred label on a regular basis. SaneBox removes this effort by re-inserting these emails into your Inbox at a time you define. Brilliant.

And oh by the way, the creator of SaneBox is on Tumblr.

reMail downloads all your email to your iPhone and lets you search full-text at light speed

Letterbox is a plugin for Apple's Mail.app that takes advantage of your widescreen monitor. It rearranges the interface into three vertical columns.

EagleFiler in a nutshell: 1. Main WindowWith a single keypress, import Web pages, mail messages, or any file on your Mac. 2. EagleFiler stores them in an open format: regular files and folders that are fully accessible to your other applications. 3. Optionally: encrypt the files, add tags, notes, color-coded labels, and other metadata. 4. Browse by folder or by tag, or use the live search to find the information you need (faster than Spotlight). 5. View, edit, or create documents directly in EagleFiler’s streamlined interface, or double-click to edit using another application.

The revolutionary enhancement that transforms Apple’s Mail into a powerful email organization system.

MacOS X's Mail and Address Book have large AppleScript dictionaries which allow almost every aspect of these programs to be scripted. Since some features are rather cumbersome in the standard implementation, I decided to write some scripts to ease workflow.

I’m using a fairly simple mailbox structure in Apple Mail. In each of my IMAP accounts, I’ve created an “Archive” mailbox. Once a day or so, I move all the unflagged, read messages from each inbox to the corresponding Archive mailbox for that account. Here’s an AppleScript that automates this inbox sweeping. It assumes that every IMAP account (including .Mac accounts) has an Archive mailbox.

"For designers who create and send email newsletters for themselves and their clients." - A service that gets great reviews across the web.

Here's how to configure Firefox 3 to use Gmail as your default mailto: application handler.

Who sent you the most email messages last year? What hour of the day do you receive the most new messages? Which of all the mailing lists you're on are the most active? A new command line tool called Mail Trends works with Gmail over IMAP and can give you

Bongo is an easy-to-use mail and calendar system, offering a simple yet powerful user interface. The goal is to make sharing, organisation, and communication simpler, quicker, and more useful."