Who knew becoming a ninja could be so affordable? Apart from the black outfit you made from scrap linens and black socks, all you really need are the tools of an assassin, and these Ninja Stealth Throwing Knives ($7) should work just fine. Arriving in a three pack with a nylon case, they measure 6.5-inches overall, with a circle loop on each end, and a blade that's reasonably sturdy yet dull enough to make them great for beginners — so you might want to sharpen them before trying to hurl them through another man's skull.
When shaving every possible ounce of weight out of your pocket or pack is really essential, nothing beats the Baladeo 34 Grams Knife ($35). Made from 420 stainless steel, this diminutive blade is true to its name, weighing in at a scant 34 grams — 1.2 ounces — all while packing in a locking system and integrated belt clip.