The text editor TextMate comes bundled with an “Edit in TextMate” input manager which allows you to call Textmate as an external editor in Cocoa apps.

If you select the Install “Edit in TextMate”… action located in your TextMate bundle (using the gear menu in the status bar), you will get a full explanation and a button that creates the required symbolic link for you.

Then, when you want to write a reply or compose a new message in Mail.app, just hit the keyboard shortcut and get to work:

When you are done, Command-S saves your work and inserts it in the message:

I have also found this useful when working in other apps. For example, if you are editing a 37Signals Writeboard , Command-Shift-S offers you a quick way to save your work off as a text file on your hard drive.

UPDATE: The TextMate people picked up this post on their own blog and offer some extra nifty tips on how to get the most out of TextMate as an external editor in Mail.

[From the TextMate mailing list and Manual ]