True story: upon playing my first computer FPS, I thought that the default WASD movement keys were a mistake so I switched changed them to the arrows. Turns out, I'm an idiot.
Deviant Artist Midnight Angel put together an arm blade stylized after the ones used in the Assassin's Creed video game series. Using a drawer opening mechanism, a spring for a screen door, and a throwing knife, the weapon works surprisingly smoothly. The full instructions are on Angel's page, and you can see it in action below.