Minecraft wiki entry for Development_versions, associated text: Debug controls (from the pre-re-uploaded version of Beta 1.8 Pre-release):
F9: Freezes the camera in its current position.
F6: Enables a strange "bouncy" movement. It locks the player's y coordinate (meaning they cannot jump or fall). It makes the player accelerate relatively slowly when moving. Additionally, the player can also clip into blocks slightly - at a high enough speed, the player can push all the way through a block. Transparent blocks have no clipping whatsoever.
N and M: Changes the view's rotation clockwise and anti-clockwise, respectively. Hold both keys simultaneously resets the rotation.
Y and H: Decreases and increases the field of view, respectively. These keys seem to affect the field of view differently to how the FOV option does - the player's hand (or anything the player is holding) will also become stretched as the field of view increases or decreases, which does not happen if the player changes the FOV option in the menu. Holding both keys simultaneously resets the field of view.
I: Makes the player look up, J looks left, K looks down, and L looks right. These movements are normally done by moving the mouse. This setting cannot be reset by holding any keys simultaneously.
U and O: Moves the third-person camera forwards and backwards, respectively. This setting cannot be reset by holding both keys simultaneously.