If this doesn't sound like much to you, chances are you've already got a system for taming an unruly in-box. I've had more than 10,000 messages sitting in my in-box at times -- so, for me, Nostalgy is a big reason why I've been able to adhere to one of my 2010 New Year's resolutions: keeping my in-box uncluttered.
In some cases, automated filters take care of this for me -- for example, I can send messages containing coupons or other shopping deals to a folder that autopurges after a few weeks. But many messages arrive first in my in-box, where they need to be moved manually to folders for short- or long-term saving.
This is where I find a single keystroke -- such as md for "move this to the Misc folder that autopurges after 30 days" -- vastly superior to dragging messages to the appropriate folder, where I can often misjudge and drop them into the wrong place.
After installing the plug-in, you can access Nostalgy's setup for key mapping in the Thunderbird Tools menu, then select the Keys tab. Choose whether you want a key to open a folder, copy to a folder, move to a folder or move a message to a folder, and then open that folder. Select the folder you want and type the key for mapping. Done.
While Nostalgy's shortcuts sometimes work slowly on my computer (I've got a lot of messages saved), it's still less annoying than numerous drag-and-drops.
Nostalgy also lets you create rules that don't run automatically but simply display a suggested action when it finds a match. You can then use a shortcut key to accept the rule's action. This is handy if, say, you want to take a look at a message subject line before it goes to another folder. The rules can include regular expressions such as a message subject including five digits and an optional dash with four more digits (i.e., a zip code), which Thunderbird's built-in filters cannot.