Super small example of using offscreen render targets to achieve a portal effect in Unity
Please see this accompanying blog post http://tomhulton.blogspot.co.uk/2015/08/portal-rendering-with-offscreen-render.html for an explanation of the techniques involved.
WASD - Camera forward/backward/strafe
EQ - Camera adjust height
Right Mouse Button + Mouse Move - Look Around
C - Toggle between main camera and portal camera view