Pocket-Mirror is a simple but extremely convenient application with one single purpose: simply have a mirror in your pocket.
Sometimes simple is beautiful and this is exactly what Pocket-Mirror is.
One of the key advantage of such application is that it always use the front facing camera, which means that you do not need to manipulate your main camera application to switch back and front the camera to use. So your main camera application will be ready 99% of the time when you need to use it.
Another difference with the usual camera application is the picture taken with Pocket Mirror will be saved, exported exactly the way as you see them on your iOS device screen, whereas the camera app will be saving the picture flipped out.
So yes simple but extremely useful, and the application is FREE, so go download Pocket-Mirror from the AppStore right now.