Super small example of using off-screen 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.
UPDATE (2019/01/28): Added touch screen camera controls when running on device.
UPDATE (2019/01/21): Added teleportation (so moving through a portal will spit you out the other side).
Note: There’s an artefact you’ll encounter when moving through a portal where the near clip plane of the camera will clip with the portal texture quad. You wouldn’t get this if using the portal stencil buffer technique but unfortunately with render buffers there’s no clean way around it. The logic can still be applied if you’d like to use it elsewhere.
WASD - Camera forward/backward/strafe
EQ - Camera adjust height
Right Mouse Button + Mouse Move - Look around
C - Cycle between main camera and portal camera views
One Finger, Hold + Move - Look around
One Finger, Double Tap + Hold - Move forward
Two Fingers, Pinch - Move forward/backward
Two Fingers, Pan - Move up/down/left/right (Camera space)
Three Fingers, Tap - Cycle cameras