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Spy Camera

You can put this camera on practically anything.
Stick the camera on something you're aiming at by being close to it and pressing your secondary attack button with your crowbar out.
Press Numpad_0 to look through the camera.
Press Numpad_. to get the camera back so you can place it somewhere else.
You can replace the weapon that needs to be out by opening the E2 and replacing weapon_crowbar with your weapon of choice.

Don't do anything illegal.