the 'coolest weapon' i've ever heard of is the human brain! if used at even close to its potential capacity, the possibilities are limitless...
Taskmaster from the X-Men comics (well, he might have originated from somewhere else, but I was introduced to him through X-Men) uses a yo-yo, too.
Vesa Toskala's glove hand. If you're standing behind that sucker (because it sucks) you can expect a face full of hockey puck.
I don't know if this qualifies but the mech from district 9 was super awesome along with all the other alien weaponry from that movie.
Ye mechs are pretty awesome. Wikus' alien hand was pretty cool too. I like quarterstaffs, they look so harmless but are so deadly.
Gravity Bomb. As seen in the anime, Kiddy Grade, pretty much self-explanatory. You're not moving off from the ground if you're near an active one.
Enough with these 3-five word responses! How about a weapon that nobody has thought of? Where is everyone's ingenious?!?! (lol). Heres an idea I thought of: A giant cleaver-like blade (similiar to the blade Cloud used in Final Fantasy) that has no sharp edge and is used for blunt, forceful attacks only. The blade when pulled from its steel sheath/scabbard turns out to be a huge, two barreled rocket launcher.
ROFL So the "blade" is a rocket launcher and when its "sheathed" its a bludgeon? How about throwing cards? Razor sharp throwing cards... with tiny diamond saw teeth on the edges!
Mel Gibson... the Road Warrior is a weapon in itself. Too bad he's also an Anti Semite. As for real life, I heard there's a knife that, when used to stab someone, shoots a balloon into you and it inflates to the size of a basket ball. Not sure if they're real of not, but that's a pretty damn good weapon.
Always thought guns that could be fired from the heel of your foot were pretty neat. Such as these ones, in Bayonetta.
When I used to play Halo, the energy swords always scared me to death. In fact, I'm sure if I played again and still saw a giant tall blue alien charging towards me with an arm of sharp energy, I'd freak out. BUT, when I had those energy swords, I always felt invincible. Quite good for scaring a random assailant, if you just pull this baby out:
Cool weapons? Oh yeah. A kukri is a very neat hand-blade, and surprisingly thick. Good for chopping. Blowguns are fantastic. They're also a lot more accurate than you might think over short distances. Throwing sticks -- the ones that are shapes just a bit like a long spoon. You swing them in order to hurl stones over a long distance. Like blowguns, they're a bit exotic but also more useful than you might think. Rods from God -- the tungsten or steel telephone poles with rockets on one end and a GPS in them to guide their fall. They strike from orbit with enough kinetic force that the resulting explosion can be the size of a small nuke. Bows are awesome. Also fun. Cigarette guns -- once used by spies. They shoot a single bullet. Bullet goes one way, gun flies the other. Concrete stakes. Seriously, they're steel, around 3/8ths inch thick, blunt, and you can swing them like a sword. I would love to use one of them (perhaps with the non-pointy end covered in duct tape to make a better grip) in a zombie story someday.