Red Lanterns' (DC Comics) Power Ring replaces the functions of their heart. While this makes them immune to any attack that would normally target their heart, they will die if they ever remove their ring.
Karl Ruprecht Kroenen (Hellboy) possesses a clockwork heart, which he can crank up...
...to enhance his speed and reflexes...
...or fake his own death.
Senator Steven Armstrong (Metal Gear Rising: Revengeance) has a nanomachine-primed metallic heart.
Kū (Naruto) implanted an artificial heart into himself, extending his wavering life.
After having his heart stolen by Chinese medics, Chev Chelios (Crank) got an artificial one which he must regularly give electrical jolts to stay alive.
The Heart of Jong (Xiaolin Showdown) give an inanimate object a artificial heart plus bring it to life