Personally, I'd like to see:
1) The AIDS bomb
Okay, imagine a bomb filled or coated in little soluble and biodegradable shards. Each one of them is filled with an infectious dose of the HIV virus. It's deployed like a normal bomb, except you make sure your soldiers aren't there. It can also be deployed like a grenade for easy ground use, or if you want to sell it to Africans and Arabs on the black market for a slightly cheaper price- and the best part is that since they mainly kill each other it'll help in wiping them off the face of the Earth.
It's like biological warfare, except better. You give it to rebel militias and rogue governments, who use it on enemy combatants and civilians, and then you give it to their enemies so they all end up giving each others' populations HIV. The best part is that normally they won't be able to afford medical care, so they'll spread it to hundreds, if not thousands of people before they know they're infected.
There'll also be a lot of shards filled with the stuff, so anyone who steps on it barefoot or pricks themselves in it will risk getting infected as well. Not to mention, AIDS is the perfect virus for infection because it'll end up killing more people over a long period of time rather than Ebola or smallpox, and not as contagious in the sense that it'll end up spreading to the West.
Any way you use it, lolaids
2) The fibrillator
Imagine a large fork, except it has a high voltage transformer and a shitload of D batteries in order to deliver a powerful electric shock of, let's say, 50,000 volts. When contact is made with a target, it will discharge, delivering a powerful electric shock. Suppose the thing is about, the size and a half larger than a normal stun baton, except the prongs are 9 inches to a foot apart, and made of steel.
The whole point is a direct blow to the chest in order to induce a heart attack. It won't be an effective hand weapon, but it's a great way to deal with people on the ground :D
3) Exploding gas grenade
Imagine a bomb that rapidly flammable gases into a room or enclosed area, and then ignites it rapidly. Suppose you have a tight spherical grenade, like a small bowling ball, filled with liquid methane or ethane, surrounded by liquid nitrogen, then a layer of explosive material followed by a small metal core filled with some kind of igniting agent or other explosive.
When deployed, the outer explosive layer explodes first, outward, leaving the core undisturbed, but propelling the methane and nitrogen outward at a rapid rate, filling the area with explosive gases almost instantly and uniformly. The nitrogen serves as an inert buffer to prevent it from prematurely exploding in order to cover more volume. Then, the second explosive ignites it after it's spread as efficiently as possible. I could suggest better compounds to use but I'm not a chemist.