Prosthetic Leg, from just below the knee.
It's a pretty awesome one, although it trades off in ankle unit flexibility for the incredible strength and durability.
It also locks on, so you don't have to worry about it coming off if you decide to kick your front door in cause you were a dumbass and left yer keys on top of the fridge...
You need a kickass build, just call Dr Aziz Nasir (They call him "Ozzy") down at MOS Services, and he'll do ya up right.
Frankenstein got nothin on that guy.