Mac user and GTDer Julian Schrader turns email messages into tasks with a keystroke, using the iGTD software:
I use iGTD to store and organise my tasks -- it is an amazing application and provided for free. I recommend it to any Mac user -- I just hit F6 in Mail and it copies the active email over into a new task created in iGTDs Inbox, a link back to the original email included.
Julian's one keystroke task-with-an-email-link-back made me drool for something similar in Gmail. He's also got 8 other good email processing strategies, too.