Mail.app has a bunch of plugins that are available to it, not like Thunderbird, where Mozilla holds a repository of Plugins, Apple doesn't do that. But there are a ton of them available on the Internet and it would be great if Apple would do something like that (like they are about to do with html5 extensions for Safari). Mail calls these plugins "Bundles" and are found in the ~/Library/Mail/Bundles directory. I just wanted to write a post about a few of the Bundles that I use with Mail.app to make my email a lot easier to use.
1. Mail Act-On
Mail Act-On, written by indev software, the same people who provide MiniMail and Mail Tags (two other great bundles that I don't use), is an Email organization tool. Basically it allows you to tie Mail.app rules to keystrokes. So for example, one of the Keystrokes that I use is "`1" (Backtick, 1). The rule I have tied to that command is to move whatever the current email I have highlighted to a certain folder. What I do is have most of the email that I deal with from listservers go directly to folders (on the server), and then the Mails from certain webservers and other "To Me" email goes to my Inbox. Since I use the Inbox Zero method of filtering email, I can read an email, and if I want to file it away, I use the keystroke to move it to my Archive folder. Simple, done. I can color emails certain colors, I can move emails around, etc. It's nice, and I suggest it's use.
Widemail is a bundle that displays your email in the three column format. Similar to how the newer versions of Outlook and Entourage display your email, I find this method of email is easier to read (from left to right) as opposed to the old Outlook method of from the "Top Down". It also allows you to color code rows two different colors so it's easy to spot where your cursor is at.
Quotefixformac is like Outlook Quotefix. It reformats emails for bottom posting, cleans up the cruft, removes the signature from the original message, cleans up unnecessary lines, and even prune replies above a certain indentation. It's a nice tool and I use it to format emails the way I like them as well.
So, just three plugins I use for Mail.app, check them out, give them a shot, support the developers that made them.