For ArcGIS Desktop, the ability to send and receive an extent. A user could send the current extent to another user. This is outside the .mxd. Maybe a button that either produces a link script, much like a web link? Or push directly to a bookmark (send bookmark to someone, though I like the idea of sending the current extent more).
Would really be slick if the receiver could drag the link onto the map, the map would then zoom to the extent.
As this is essentially leaving the current .mxd, projection information should also be sent. That way, the receiving user can be in any projection and have the extent reprojected.