I recently ran into this neat little feature. Not sure if it existed before but...
In Mail, if you hit Command-I, you'll get a useful summary box showing statistics for the account containing the currently selected message. In my case, it popped up all the information about my .Mac account, including the capacity of the mail server and how big each folder is. This is very useful to see where all your IMAP or .Mac space is going.
[robg adds: I had completely missed this very useful feature. Why? Because it seems Mail has broken one of the more long-standing Mac covenants: "Thou shalt not have features hiding in contextual menus that don't also exist in the main menus." There is absolutely no reference to Account Info in any of Mail's menu items. The only place it exists is in the contextual menu you see when you control-click in the mailbox panel. And even as listed there, there's no keyboard shortcut shown, so you wouldn't know to hit Command-I. Bad UI decision, Apple. Please don't start hiding menu options on us ... this is common in Windows, but has been (thankfully) nearly absent from the Mac for forever, and it's one of the things that makes the platform such a joy to use.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050503171819208