So you like working with Apple Mail? Did you know that you can fully customize the client with all sorts of plugins, add-ons and scripts? Most of them are easy to set up and can make your Apple Mail a bit more powerful and original than it normally is. The Hawk Wings website seems to be an expert on the matter and lists out more than 140 add-ons, plugins, mail templates and other apps that work with Apple Mail. Although a few are shareware, the majority are free and should only take a couple of seconds to install.
You'll find plenty of notification tools for unread messages like GMail Notifier or MailAppetizer. Hawk Wings also lists apps that add extra functionalities to the mail client, like the Apple Mail Signature Generator or MacResponder, which sends auto-replies. The Mail.app being an Apple product, it integrates easily with other applications, particularly Apple ones like Spotlight or Bonjour. Hawk Wings lists plugins that work with other programs such as Apple Mail Conduit to sync mail between the client and your Palm device or Daylite Mail Integration which integrates Apple Mail to the Daylite project management app. You'll find some more switching tools to move from another client like Eudora to Apple Mail, lots of iCal and Address Book plugins, archiving and spam tools.
If the appearance of the program and your emails is what matters to you, you'll also find many interface tweaks and lots of Leopard templates to send original messages to your friends.
Well worth taking a look at, this list should provide some useful add-ons if you're a big fan of the Apple Mail client.