I'm sure a lot of you already know this, I bring it up only because I see it asked every now and again, and usually I see people describing how to do this the hard way (that is, manually with + and - offsets). But it doesn't always have to be so tedious! I think the picture pretty much tells it all, if you have made one side of something symmetrical, and you need to duplicate it but in reverse (like a wing for a plane or something more sophisticated than this poser hand) Just duplicate the object or group of objects, and flip it! Voila, mirror image of your model. It's great for things like metaball arms and such too. May it speed your modeling.