I'm not a strict Getting Things Done / GTD follower, but I do try to keep my inbox from becoming my todo list. It's easy to slide into that habit, but the inbox can't tell you anything about deadlines or priorities, so I really like to have a task management program outside my mail program, where I can line up tasks based on something besides when someone sent me mail.
I use Things from Cultured Code in order to manage my tasks, and I use Mail.app as an email client. I get mail rapidly enough that the simple act of creating a task in Things in response to a mail takes a significant amount of time each day - a great target for automation.
My first goal was simply to have a keyboard shortcut that would pop up a quick entry task creation dialog for Things in response to an email, while in Mail.app. That's easily done, but while we're automating why not make things significantly better? My definition of "better" here is to create the Things task with the email subject used as the task name, then the task notes prepopulated with a hyperlink to the email, made in a way that is stable no matter where the email is saved. Finally, I want the email automatically moved to an archive folder in Mail.app so my inbox is decluttered. The final bit of glue is the Mail.app plugin "Mail Act-On" which lets you create rules triggered by keyboard shortcuts.
I have a Mail Act-On rule set up in response to Ctrl-T that runs the Applescript I use to automate things, then moves the message to a folder.
All pretty easily done, and something I use all the time. So much in fact that I'm sharing it here - use and enjoy: NewThingsToDo2.scpt