While compiling the Hawk Wings Plug-ins and Add-ons List, I came across three two one well-known plug-ins that don’t work in Tiger (or aren’t needed so much), won’t be updated and look likely to fade away. So, a quick eulogy to old friends.
MailEnhancer is very much alive. MailEnhancer gave us signature matching for specific accounts, made the dock icon count all unread messages, not just those in the inbox, and launched the Activity Viewer on a manual mail check. It was the first ever plug-in I installed. (Sebster writes in the comments to this entry that reports of MailEnhancer’s death (by me) are greatly exaggerated. It works fine, although the developer’s website is down. Excellent! Get it from versiontracker.)
MailTemplate is back. 
MailTemplates offered users the ability to use pre-made messages or templates to send similar messages to different people. In its most recent version it also included some extras for the contextual menu which were very useful. It doesn’t work correctly in Mail 2.0 and the developer has announced that its development is discontinued. That’s sad, but leaves a hole in the market for an enterprising coder.
Mail Priority allowed you to set the priority level of messages, to request “mail received” receipts from the people you emailed and colouring of messages via the contextual menu. It doesn’t work with Tiger and the developer hasn’t said that he has any plans to update it.
There must be more. Old friends, gone but not (quite) forgotten.