Mirrors are a great tool for experimenting and changing the feel of a room.  By inroducing a mirror to a rom you can instantly make it appear larger, lighter and more homely.  Depending on the style and type of mirror you can really do wonders to a room.

Take for example a hallway.  By adding a mirror near the main entrance to the hall where a light source is you can instantly add more light to what would normally be a dark room.  This is because the mirror can bounce the light against the walls better than the natural light can manage on its own.

Another example is to take a large oak mirror and add it to a wall in your lounge or living room.  Provided it is placed in the right way you can make the room appear bigger than it actually is and is ideal for smaller rooms in the house, a nice oak mirror can also have the advantage of making the room look nicer and more homely in addition to the mirrors standard purpose and use.

The final example like the first example focuses on light and what a mirror can achieve.  By adding a mirror to a room and positioning light sources whether that be natural or artificial close to the mirror you can create a mood in a room and change the way the light falls in the room especially in the evening when there is little natural light.  A tall lamp next to a mirror can light up a whole room reducing the need for extra lighting and ultimately saving cost of electricity.

Think about adding a mirror to the next room you decorate and see what effects you can make.