Braided Fishing Line
Braided fishing lines have been around for a long time, but it was the category known as "super lines" that were a revolution in fishing line. Gel-spun-polyethelene and other materials featuring incredible breaking strength allowed for increased casting distance due to their very small diameter. They also have very little stretch resulting in senitivity far away from their monofilament counterparts.
They also resulted in the ability to use much smaller tackle due to the amount of line you could pack on a reel. You could argue that braided fishing line spurred a lot of the growth seen in both spinning and casting reels as both had to get stronger to handle the fish that braided lines were capable of catching.