You now might use saltpeter if you want an old stump in your yard to decompose faster, but for a millennium it was the cause of wars, massacres, and great grassroots scientific efforts. It changed the history of the world.
Saltpeter, or saltpetre, is also known as potassium nitrate. It's simply a potassium atom, a nitrogen atom, and three oxygen atoms. The fact that it gives its three oxygen atoms away fairly readily is handy for things like decomposing vegetable matter, but for most of human history, it was hardly something that could change the world. Then, around 1000 AD, some inventor in China discovered that mixing saltpeter with charcoal and sulfur made a bang. It wasn't magic. Ordinary things don't burn explosively fast because fire needs oxygen. Oxygen is only readily available where the object meets the air, so fires are slow. Potassium nitrate, mixed in with the burning substance, provides oxygen everywhere, so every part of a substance explodes into flame at once.
As explosive weapons became more and more important in war, control over saltpeter was crucial for any country's grip on power. Saltpeter, for a long time, could not be manufactured from scratch. Anyone who wanted it was dependent on natural sources. These resources were either caves where groundwater builds up crystals of the stuff or fields of ancient and decayed matter where bacteria in the soil slowly, over time, added oxygen to nitrogen.
By the early 1600s, European powers found ways to harvest saltpeter, generally by blanketing large areas in decaying vegetable matter and sprinkling them with urine, which provided the nitrogen. The process took two years, and didn't provide nearly enough gunpowder. And so, the fate of nations came to depended on who got access to natural deposits of saltpeter. The early winners in that game were Spain and Britain. Spain laid claim to Chile, which had huge dry saltpeter flats near the Atacama desert. These supplied the military needs of an empire. Later, they were deemed just as critical to the needs of a country. One of the worst tragedies in Chilean history, the Santa Maria School massacre, happened in 1907 because saltpeter miners went on strike. The Chilean army attacked, killing 2000, breaking the strike, and securing the supply of saltpeter. The massacre is still mourned and commemorated by many, including the modern Chilean government.
Britain got hold of India. India had such vast caves of saltpeter that it was still the primary provider saltpeter in the 1900s, when a manufacturing process was invented. India's resources and market domination put other miners of saltpeter out of business. This meant that, when a country found itself at war, the course of the war was in part determined by whether or not the country was able to trade with India.
The United States realized this, to its cost, in 1812, when a war with the British cut off the nation's supply of saltpeter. What followed was an inefficient and unpracticed, but frantic, attempt to mine saltpeter from natural deposits in Kentucky and other caves in the American south. (The Great Salt Petre Cave is now a national park.)
The situation repeated itself during the Civil War. The blockaded south turned back to mining saltpeter-rich soil around caves. When that didn't provide enough saltpeter, scientists stepped in, trying to get a grassroots chemistry movement going. They circulated papers teaching people how isolate saltpeter from local caves, and produce it by saving up their own urine.
The era of saltpeter as a critical natural resource ended when German chemist Fritz Haber found a way to harvest nitrogen from the atmosphere. Today, the Haber process is most famous for providing people across the world with a ready supply of nitrogen fertilizer. It's justified fame, as it increased crop yields enough to feed hundreds of millions of people. But the Haber process also changed the political and scientific make up of the world. It made saltpeter into a boring chemical, instead of something to go to war over.