Brass Knuckles: Long a mainstay of juvenile delinquents and tough guys, the brass knuckles on this case are actually aluminum and intended to be used as a handle. Yes, the manufacturer, Knucklecase, specifically states that "Any other use ... is considered a misuse," so don't get any funny ideas. It's also totally recyclable. There's a similar version, Nukles, constructed from a rubber/plastic composite and available in three colors, as well as a soft silicone version or those of you who are more lovers than fighters.
Pepper Spray: If you like your accessories to be multi-functional, then check out the the SmartGuard - an iPhone case that also houses a patented pepper spray canister and is available in three colors. While holding a tube of pepper spray up to your face might sound like a terrible idea, the SmartGuard has a safety tab that prevents it from accidental discharge; indeed, to even use the pepper spray you'll need to snap it out of the case first. However, if you ever find yourself needing a 6.5 second burst of 5 feet worth of pepper spray, you'll surely be glad to have it. Replacement canisters can be purchased as well.
Revolver: Designed by Thingiverse member Juniortan (who has also created a Katana Umbrella) using a 3D printer, the Revolver iPhone can tuck nicely into board shorts - and will likely NOT make you friends with local law enforcement. However, it is kinda cool to whip out a revolver to answer your phone and would make an outstanding addition to say, a cowboy costume for Halloween.
Studs: This Sid-and-Nancy option is a DIY endeavor - just buy wicked looking studs, mark when you'll put them on the case, then screw the studs through and you'll be the punkest kid on the block. You'll also probably never hold your phone comfortably again, so perhaps the better option here is smaller pyramid studs. There's a nice white version at LSD Fashion Lab, and Etsy has over 800 results for "studded iphone case." There's even a softer silicone version from Rebecca Minkoff, who also makes the rubber brass knuckle case.
Kevlar: A case that's more about the offensive than the defensive is the Battle Case from Strike Industries, which features a loop at the end of the case so you can easily pull it from a pocket or purse. The Battle Case also makes it easy to attach the case to a carabiner or belt lanyard so you have more options for carrying your phone. The stand-out option here is the upgrade that include Kevlar inserts and lens protectors for a truly hard-core (but not bulletproof) case. (Images: Etsy seller WallSparks, Nukles, SmartGuard, Thingiverse, FujiFiles, Strike Industries)