LinkABoo is a stand-alone app that creates and manages hyperlinks to emails in Mail.app.
It only works with POP messages and it only works with Tiger Mail, but with those restrictions it allows you to create links back to your emails in any other app that supports hyperlinks. Nifty, eh?
This is how it works.
Select the email that you want to create a link to. Launch LinkABoo (if you were doing this a a lot you would want to create a Quicksilver trigger for it) and it presents you with a dialog allowing you to drag the hyperlink into your other app of choice or to copy the link to the clipboard.
Then with the click of your mouse you can open the mail from the other app.
For example, you can dump the hyperlink into an iCal to-do’s URL field (I suppose that there are some people in the world who don’t use MailTags which is even smarter about iCal-Mail integration).
Clicking on the URL field then opens the email to remind you what the to-do or event was about:
The first time you click on the link there is a small delay as LinkABoo consults its SQL database (stored in the ~/Library/Application Support folder) and then finds the matching email. This will also happen if the email is moved, although LinkABoo finds it again fast enough.
You can also use it to hyperlink to emails from a personal information manager app like Yojimbo 
(or a raft of others):
Or a “Getting Things Done” (GTD) app like EasyTask (or a raft of others):
The possibilities are endless.
LinkABoo is currently free and in beta. You can get it from the developer’s web site .
Based only on the screenshots here, it looks as if the app creates its own “linkaboo://” URL type. This is both needlessly restrictive and redundant, since we’ve already got the “file://” URL type. Since Tiger stores each Mail.app message as a separate file, the “file://” URL type would A) work just as well; and B) work for any other file in your system.
Sure.
I guess the advantage here is the ease of use which the restrictive and redundant front end provides for people who want to link stuff together.
[...] Original post by Hawk Wings and software by Elliott Back [...]
[...] A week ago I posted about linkaboo, an app that provides a framework for linking to your emails in iCal, personal information managers and many other apps. [...]
LinkABoo uses the linkaboo:// protocol, because the file:// protocol would not do the job. If a mail is moved inside Mail.app, the file:// path will stop working, as opposed to the linkaboo:// link, which will locate the moved email.
OK, that makes sense. I withdraw my earlier sniping.
[...] The new version also provides for hyperlinking within items. You can add web URLs or links to emails with LinkABoo or (soon) MailTags and add links to files and folder just by dragging them onto the hyperlink dialog: [...]
[...] If you’re a POP person, the next public beta of MailTags will work for you too. Or if you can’t wait, LinkABoo makes solid links for POP messages. [...]
Stand-along?
I just right-click (Cmd-click for those of you with one button) on a message in Mail and select ‘Copy Message URL’.
Example URL: message://09e101c78d5e$377b53e0$0248a8c0@NS1 which is arguably better than something dependent on a plugin.