What is this effect ?

In this image You can see that on edges image is much darker than in middle. I only want to know how to call this effect.alt text

Its vignetting.

And you can make it by following any of these tutorials.
http://mobilegamedev.tumblr.com/post/48833277558/easy-fake-vignette-effect-for-unity-free-using-ngui
http://armedunity.com/topic/5570-vignettechromaticblur/
http://unity3d.com/learn/tutorials/projects/stealth/screen-fader

It’s called vignetting.