Ballon Bombs - Yes, you're reading that correctly - according to the 1898 Hague Convention, it is against international law to drop bombs from balloons. Japan famously sent scores of balloon bombs to the American Pacific Coast during WWII, with the purpose of causing forest fires. While most landed harmlessly, one did cause casualties - a balloon that landed in a forest near Bly, Oregon, that exploded and killed a Sunday school teacher and five children. The practice of shooting a rifle or dropping a bomb from a balloon is still technically forbidden to this day.
Bat Bombs - In the second world war, Americans experimented with a secret weapon designed to decimate Japanese cities. At the time, most of Japan's cities were made of wood and paper. The idea was to release a bomb filled with sleeping bats (captured from caves in New Mexico), wearing collars containing a napalm-like incendiary. Upon release at dawn, the bats would disperse and roost under the eaves of Japanese homes up to 40 miles away. The project, code-named "X-Ray," was tested in 1944, but the war effectively ended with the bombing of Hiroshima and Nagasaki. It might sound funny today, but testing showed these unusual weapons to be tremendously effective...some say even more so than the A-Bomb. Today, bat bombs would certainly be prohibited under Protocol III of the Convention on Certain Conventional Weapons.
Bio-Weapons - Believe it or not, bio-weapons are some of the oldest terror weapons known to man. They date back to at least the days of the Mongols, who would catapult rotting, infected bodies over castle walls in order to spread disease and sickness. It's also been suggested that the Black Plague, spread by fleas on the back of rats, and originating from Asia, was the lingering result of a primitive bio-terrorism attack from centuries before.
Blinding Laser Beams - This might sound like one of those sci-fi things that would never happen, but the technology's been around for 40 years. "Blinding" laser beams don't refer to the laser "dazzlers" that police and special ops teams use; those are low-powered beams that aren't designed to cause permanent blindness. This ban refers to lasers powerful enough to cause permanent blindness, which is amazingly easy to do, as most juvenile delinquents with laser pointers have been warned. The prohibition against deliberately blinding weapons goes way back to some of the first weapons bans passed in the 19th century.
Dirty Bombs - Bombs laced with radioactive material are forbidden under international law, though most countries wouldn't bother with them anyway. The point of a dirty bomb is to irradiate an area and make it uninhabitable -- which means that the "winner" of the war can't go there either. That aside, the amount of radioactive material necessary to make a dirty bomb effective could just as easily be used to build a full-on nuclear bomb.
Flamethrowers - According to Protocol III of the Convention on Certain Conventional Weapons, flamethrowers aren't explicitly forbidden on the battlefield, provided the battlefield is nowhere near civilians. Mostly, this protocol refers to incendiary devices in and around civilian areas. It doesn't necessarily prohibit the use of flamethrowers in, say, an open tank battle or clearing caves in Afghanistan. But most guerrilla fighters hide behind or within civilian areas. If they're using human shields or might have captives flamethrowers are a no-go.
Hollowpoint Bullets - Hollow-point bullets (aka "expanding ordinance") were explicitly outlawed for use in international warfare by the Hague Convention of 1899, which was, in fact, only a continuation of the St. Petersburg Declaration of 1868. This declaration forbade the use of exploding or expanding projectiles of less than 400 grams, which drew a clear line between "bullets" and "artillery shells." The concept behind the ban was to avoid using bullets that "made death inevitable." Which, some might say, is the whole point of shooting someone in the first place.
Locust, Fleas and Rats - It's been done, and to sometimes devastating effect. The Black Plague is theorized by some to be the result of a lingering bio-terror attack from Asia. Today, using hordes or plagues of animals carrying disease in war would be completely illegal.
Microwave Lasers (limitation) - Yes, laser cannons are a real thing, and they've been around for quite some time now. Today, the Air Force uses massively powerful laser cannons mounted to aircraft and battleships, which can use them to shoot down incoming missiles from up to 250 miles away. Hypothetically, they could be mounted to tanks and used to incinerate human targets on the ground - but such use of directed energy weapons is currently forbidden, in large part because too low a dose from too great a distance might not kill the target so much as cook their eyes, which would be a violation on the ban against blinding lasers.
Mustard Gas - The terror of the trenches in World War I, mustard gas gets its name from its yellow-brown color and its odor, which is apparently similar to horseradish. Because it's heavier than air, mustard gas proved particularly effective in clearing trenches, and was almost single-handedly responsible for the 1928 Geneva Conventions. When inhaled, the gas causes the lungs to fill with fluid, essentially drowning the victim in their own fluids.
Napalm - You might love the smell of napalm in the morning, but the same Protocol III (passed after Vietnam) that restricts the use of flamethrowers also limits the use of napalm. It can't be used anywhere near civilian targets, nor can it be used to burn down forests unless the trees are being used to conceal military combatants or vehicles. So, napalm isn't banned, exactly, but more often than not, it can't be used on today's battlefields.
Nerve Gas - Nerve gases of all kinds have been systematically outlawed by both the Hague and Geneva from 1899 all the way up to 1993. All nerve agents (like Sarin, VX, Tabun, and Soman) work in the same basic way: By blocking blocking the enzyme that normally destroys a very important neurotransmitter. Basically, nerve agents cause your entire nervous system to malfunction, like an electrical system full of short circuits. Death generally comes as a result of a shutdown of the respiratory system, but not before painful blisters, boils, and internal hemmorrhaging occur.
Non Self Destructing Landmines - Since the Vietnam war, decades-old unexploded landmines have been a deadly menace in Southeast Asia. Cambodia has one of the highest rates of amputees in the world, as some 40,000 in its population have stepped on land mines planted during the Cambodian Civil War in 1970. In 2013 alone, some 111 people were killed by land mines buried more than 40 years before. For that reason, as of 1980, mines placed outside of fenced and cordoned areas must use some sort of self de-arming device or self-destruct mechanism set to go off after a certain period of time. Standard land mines may still be used, but can only be employed inside of fenced-in areas, away from civilian populations, and must be removed or destroyed when the conflict ends.
Phasers - There are all kinds of directed energy weapons on the table, from "death ray" lasers to sonic cannons to real life plasma rifles. However, as of right now, directed energy weapons with enough power to kill human targets are forbidden in war. This doesn't apply to de-powered non-lethal microwave emitters like the Active Denial System currently in use. ADS puts out enough energy to cause an intense sensation of heat on a large crowd, but it's not enough to cause actual burning. The sensation has been compared to standing a few feet away from a large oven with the door open. It is possible to set the ADS on "kill," but that is illegal for the time being.
Plastic Landmines - According to Protocol I of the 1979 Convention on Certain Conventional Weapons, weapons that use non-metallic fragments not detectable by X-Ray are prohibited in war. The rationale is pretty obvious, since field surgeons can't remove fragments they can't locate within an injured body. This doesn't prohibit the use of plastic and undetectable materials in weapon design, it just means that weapons can't be designed to use undetectable fragments as a primary damage device.
Poisoned Bullets - The world's oldest known arms agreement, the Strasbourg Agreement of 1675, explicitly outlawed the use of poisoned bullets. The first guns used in warfare weren't terribly accurate, so soldiers would often supplement the lack of accuracy by soaking their bullets in some kind of poisonous or infectious substance. It was not unheard of for legions of soldiers to stow their bullet caches inside rotting corpses, though the bottom of a latrine pit worked just as well. When France and the Holy Roman Empire went to war, they initially experienced a massive wave of casualties not from gunshot wounds, but from subsequent infection. More than 250 years would pass before Geneva once again addressed chemical and biological weapons.
Salted Bombs - Salted bombs are very similar in concept to dirty bombs, but are true nuclear weapons created specifically for the purpose of shorter-term area denial. A "salted" nuke contains an isotope of another substance like cobalt, gold, zinc, or sodium. During a nuclear blast, these elements become a huge cloud of fallout. These types of weapons are the same type used in the Soviet "Doomsday Device" from Dr. Strangelove. Small, one kiloton salted nukes could be used tactically and made so that the radioactive fallout decayed in a year or two, thus denying large swaths of land to enemy forces for a time. But radiation is invisible, and these weapons are generally prohibited because of their potential lethality to civilians.
Smallpox Blanket - While America in general has avoided the use of biological and chemical weapons, many historians agree that we did make at least one attempt at genocide through bio-weaponry. America's "manifest destiny" meant getting rid of the original inhabitants of the continent. Many were killed by bullets and blades, but far more were wiped out as a result of diseases introduced by Europeans. Coming from a center of worldwide trade, Europeans developed at least partial immunity to many diseases, while themselves remaining carriers. Where Europeans went, plague almost always followed, helping to exterminate native populations and assisting in conquest.
Spiked Pits - These old fashioned death traps are technically prohibited or regulated by Protocol II of the 1979 Convention on Certain Conventional Weapons. Pits with sharpened bamboo spikes maimed thousands of soldiers in Vietnam and in the Pacific during WWII. Adding insult to injury, the Vietcong and Japanese would routinely roll those spikes in human or animal feces first, causing secondary infections after even the smallest scratch. That, in itself, is a direct violation of the 1907 Hague convention on biological weapons and might even violate the 1675 Strasbourg Agreement.
Tear Gas - Believe it or not, the tear gas that police routinely shoot into crowds in America is technically outlawed for use in war by the Hague Convention. Even though it's generally non-lethal, tear gas is still an inhalant chemical weapon that obstructs breathing, that puts it in the same legal class as mustard gas. So: legal to shoot at protesters in Missouri, but not legal to drop on a machine gun nest in Afghanistan. Go figure.
Unexploded Bombs - Protocol V of the Convention on Certain Conventional Weapons prohibits the use of "explosive remnants of war," such as unexploded bombs and artillery shells. This protocol came about in the 1990s, when the newest crop of Middle Eastern jihadis began assembling roadside bombs from unexploded Soviet ordinance from the Afghanistan conflict. IEDs are remain a source of terror in that part of the world.