Start with Irukandji jellyfish. These tiny creatures (the bell is roughly 5mm on a fully grown adult) are incredibly toxic. From Wikipedia:
Irukandji syndrome is produced by a small amount of venom and induces excruciating muscle cramps in the arms and legs, severe pain in the back and kidneys, a burning sensation of the skin and face, headaches, nausea, restlessness, sweating, vomiting, an increase in heart rate and blood pressure, and psychological phenomena such as the feeling of impending doom.
And all that is without any help from ill-meaning scientists. Oh, yeah, and by the way:
the Irukandji also has stingers on its bell
...the ability to fire stingers from the tips [of their tentacles] and inject venom.
And you thought Jaws was scary.
Now, if you have millions of these tiny creatures, there are a variety of ways you can weaponize all or part of them.
The simplest way to weaponize them would simply be to fill a large-bore water gun with them and fire them at your enemies. Low-tech, low cost, and very dangerous, the tiny creatures will leave your enemies writhing in agony and doom at best. Body armor won't stop them, either; all it takes is a 5mm hole and the tiny jellyfish will sting them.
Any water toy will work, too. That's right. Squishy splash balls, sponges, water guns, water balloons, and even squirting rubber duckies could be converted into a weapon, simply by filling them with jellyfish. If you are a horrible, evil, black-hearted, soulless person, you could even send children with water guns to attack enemy soldiers. No one suspects a child with a squirt gun, at least until everyone starts screaming.
Dumping a few thousand live Irukandji in swimming pools, drinking water, even puddles on the side of the road will wreak havoc. But why bother with the jellyfish? Extract the venom for use as a poison! There are countless applications for the venom alone; as an additive to anything from drinking water to improvised explosives, the venom can make a bad day much, much worse. victims won't be stumbling out of wreckage covered in dust; they'll be thrashing on the ground in burning agony. If you want to leave an impression on the local population, that's the way to do it.
Semi-Lethal Spreading Mines
A normal explosive mine is bad, but often it leaves nothing but a corpse, and if you're in a hurry, a corpse is easy to leave behind. Instead, fill mines with jellyfish barbs. Upon release, the barbs will be flung out to attach to anyone nearby, injecting them with the venom and leaving the victims in pain, but alive.
But that's not the worst part. When that soldier is taken to a field hospital, anyone that touches him without a strong pair of gloves (leather would work; thin rubber of latex would not) will also be injected with the venom, as the sharp, springy barbs stab into them. Even if all that happens is that contaminated clothing is washed with uncontaminated clothing, there is a chance for the barbs to spread.
Thus far, everything has merely used parts of a jellyfish. But what if we apply Science™? Breeding Irukandji jellyfish in captivity has thus far proven unsuccessful, but then again, we already have truckloads of the things, so I'll assume we've gotten that part down.
Once we can breed, we can selectively breed; as fast as jellyfish reproduce, it won't take long to create an even more powerful venom, or a venom that highlights certain aspects - for instance, the pain and agony part is reduced, while the "sense of impending doom" is strengthened. Now instead of a drug that causes pain, we have a mind-altering drug. Enemies gassed with an aerosol form would be panic-stricken, even to the point of self-harm.
Other non-lethal forms could include a sprayable form, similar to mace, or a diluted form use in riot guns or water cannons for riot control. It's hard to riot for the cause when you are shivering in pain and wallowing in thoughts of doooooom.
Note: Please don't try this at home. Or anywhere else. It's terrifying, and you'll probably end up stinging yourself (which is what the scientist who discovered the Irukandji syndrome did, though, and it got named after him; who knows, you might end up famous).