Eyeglasses

The invention of eyeglasses at the start of the Current Era dramatically changed the lives of the people of Vanelle. It may seem like a small thing now, but the ability to correct poor eyesight allowed many Vanellians to work for longer, do precision jobs where they previously couldn't and opened up the wonders of reading and writing to thousands.
At first, eyeglasses were both expensive and imprecise. They were costly to the degree where only the very wealthy could afford them. In addition, there were only a few artisans able to manufacture them, leading to exorbitant prices. While some manufacturers are still making very expensive pieces, although this is now due to the materials used and the design of the frames. Glass cutters have also greatly improved in making the lenses more precise, and if you're able to afford it, some even offer personalised fittings - as opposed to the standard model lenses offered off-the-shelf.
The knowledge of how to manufacture the corrective lenses is now widely distributed among glass cutters, and so jewellers and craftsmen offer cheaper models. While they would still be expensive to a peasant or labourer, eyeglasses are now within reach of most people. As mentioned above, this has had a profound effect on the working population of Vanelle in a wide variety of professions - craftsmen, hunters, scribes, clerks, educators and many more are now able to continue working for years after their eyesight starts deteriorating. This simply wasn't possible before. More people are also able to learn how to read and write, which not only makes for a more educated population; it has also increased the demand for reading and writing materials as well as traffic to The Grand Library of Cirranne.
And that's not the end of it. The technology is still developing. Skilled glass cutters are able to make thinner, lighter lenses; frames are becoming more comfortable; soon the wearer won't even be able to feel their weight.