What technological advancements in military warfare came from the Civil War?
Repeating rifles and ironclad ships were the two big ones.
The American Civil War saw a number of "firsts": the first repeating rifles, the Gatling gun (an early machine gun), the first ironclad warships (the Monitor and the Merrimac), reconnaissance balloons, and even the first submarine, the Confederacy's HL Hunley.
Culturally, it had the first war photographers, the Matthew Brady Studio photographers, who photographed the battle aftermaths (from Antietam onward) after the fighting but before the bodies were removed.
Medically, this was the first war whose doctors and surgeons accepted Cell Pathology (as opposed to the Four Bodily Humors) as the source of disease. They were the first Western battlefield caregivers to practice palliative medicine. There were still a lot of amputations, but no therapeutic leeches or bleedings. Historically, a lot more soldiers died from disease than from battle-inflicted wounds, but the Civil War was a turning point for this.