There is a hotkey mentioned by Gleb in the Blender Guru podcast that allows the camera view to be flipped on the x/y axis, I can't remember it for the world, nor find it anywhere in google. The episode is like 3hs long so I'd prefer not to go through it again trying to find the moment he mentions it. What is this hotkey?
Give the camera a negative scale on the local X axis.
Both S X X
-1 and ⎈ CtrlM X do this (use Y instead of X for flipping vertically), or you can set the scale directly in the transform panel:
Rendered animation playback
When previewing an already-rendered animation (⎈ CtrlF11) you can flip the display horizontally with F and vertically with Shift-F.
The VSE has an option to flip strips in Properties region > Filter:
Thanks ideasman42 for mentioning these :)