The other day on Twitter, Macworld senior editor Dan Frakes mentioned that he had accidentally discovered an undocumented keyboard shortcut to delete messages in Mail. My guess is that he just leaned on his keyboard, but he figured out, after deleting a number of messages, that this shortcut is Control-H.
This makes sense, as this is the Unix keybinding for the Backspace or Delete key (see this Wikipedia article), and OS X uses these shortcuts, at least in Cocoa applications; you can use the Control-H shortcut to delete text in those applications that use the Cocoa text input framework. But its use to delete messages in Mail is interesting. If anyone discovers other apps where this works, post them in the comments.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120320062644784