Mirrors reflect, double and most interestingly take what they set their eyes on and wear it. These qualities can make it one of the single most versatile and effective decor solutions. Without a lot of rambling, here are some amazing examples :)

This strategically placed mirror breaks the brickwall to reflect and double the beautiful chandelier
A mirror on a rickly coloured wall becomes a bright spot that reflects lighter tones. This smartly placed floor mirror almost makes it look like a doorway to another room!
There's nothing like a period frame that comes from another era.
Or double it up with the clean lines of a simple frame

Many mirrors are a good idea..
Interesting shapes make interesting reflections
A triangular mirror (at my place) was a nice find.
Even a simple mirror in the kitchen just livens things up..

And here are some amazing shapes & sizes from House & Gardens just to prove what a great decorating idea mirrors can be. This is my favourite..
Anything would look brilliant framed in that.
white works wonders..
This red will defintely stop anybody on their tracks..!