Real time mirroring of a selection of objects? Suppose you're working on a room containing lots of objects, and you need an exact copy that is mirrored. This can easily be performed by selecting all the objects, then duplicating and mirroring them with Shift D and Ctrl M.

In this case, any time I make a change to this room, I would need to delete the duplicate room and contents, then perform the Shift D and Ctrl M procedure again to ensure that the room and all its contents are the same.

To make things faster and more convenient, is there a way to automatically do this? What I'm looking for here is something similar to the mirror modifier, where changes to the original piece are mirrored at the exact same time, so that you don't have to do it yourself. Unfortunately, the mirror modifier only works on individual objects, and thus, a room with hundreds of items in it would be a nightmare to mirror individually.
Take all the objects that you want mirrored and add them to a Group. (Select them and use Ctrl-g).

Then create a single empty somewhere on the mirror axis, and scale its x or y axis by -1 (I edited the scale value in the properties but s x -1 would also work.  Now in the Properties > Object panel in the Duplication section change its duplication from None to Group and select the group you created in the first step.  There should be copies of all the objects mirrored along the axis that was scaled by -1.

When you create more objects that need to be part of the mirror group you can go into the Properties > Object panel and look for the Groups section and use the Add to Group button to add it to the dupligroup.