Free Cam

You can move camera around and through blocks. Default key F6, but can be changed in options. You can open chests through walls or attack entities.
You can adjust camera movements settings here.

Acceleration: (how quickly you reach max speed)

Max Speed:

Slowdown Factor: (how quickly you slowdown after releasing key, closer to zero = quicker slowdown, closer to one = slower)

Render hands (doesn't work with OptiFine)