Glad to see quite a lot of things that are considered useful. The dots mentioned by Umesh for visually impaired people is also a point to note, and on currency, too, there is the Braille feature that may simply be considered as part of the design, but it is vital for them.
Have you also ever used a piece of paper in a useful way? When folded and made a bit thick, it can be wedged beneath the feet of cupboards or other furniture that is slightly off-balance. I have done so, as also to wedge shut a loose window net grille with it.
I would like to share some interesting information here about Lillian Evelyn Gilbreth, about whom I read in the book Women in Science by Rachel Ignotofsky and which inspired me to come up with this topic. In the section on Lillian, it is mentioned how she came up with useful things that would aid in work efficiency. Her pioneering inventions included the foot pedal on the garbage can and shelves in the fridge. It was her wonderful work on ergonomics and motion studies that led to new tools and even layouts for kitchens that would cut down on women's time in the kitchen to a few hours, such as what has come to be known as the 'work triangle' which is the what determines the distance from the sink to the cooking range. Kudos to her!
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell