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.
Altoid tin alterations are pretty popular among the hacking community, but this one takes it down in size. Using the Altoids Slim tins, Instructables user Joshua Zimmerman put together these bite-sized LED flashlights for under $5. With just a tiny bit of drilling and soldering, you could easily assemble a handful of these in an hour or two.