Another shortcut key discovered by accident. I was moving around my inbox with the keyboard, deleting messages, when I mis-keyed and hit Insert instead of Delete. Much to my pleasant surprise I found that Insert flagged the highlighted message with the default red flag. If you hit Insert on a message that is already flagged, it marks it as complete. This works with one, or many, messages highlighted.
To clear the flag completely, or to set a flag with a color other than the default color, you still need to go through the standard process. I’ve never risen to the level of using the various colors for flags, so I’m not hampered by the default action of this shortcut.
I’ve only tested this on Outlook 2003. Can anyone verify this with other versions of Outlook? How about Entourage?