Batarang

You'll likely never need to neutralize supercriminals in an overrun insane asylum, but with the guidance of an Instructable by user Spookylean, you can craft a (semi) working "Batman: Arkham Asylum" style batarang just in case. You don't even need Bruce Wayne's limitless budget either, just around $20, some tools, ten hours and some good old fashioned Bat elbow grease...