Nettie Stevens: Sex Chromosomes

Nettie Steves actually discovered that the size difference in the 20th chromosome was the determining factor in the sex of an embryo. She was the initial discoverer of this fact, but her colleague and mentor E.B. Wilson was given credit instead. 

Mary Anderson: Windshield Wipers

In 1902, Mary Anderson came up with the idea for windshield wipers after observing that cars in New York were having trouble seeing through all the snow on their windshields. She patented her idea but could not find a manufacturer to back her and mass produce her creation. Then after her patent expired, Robert Kearns manufactured her wiper blades and was credited with the invention. 

Elizabeth Magie: Monopoly

The Landlords Game was a creation by Elizabeth Magie back in 1903, and patented her idea in 1904. But no manufacturer would make it as they called it too complex and absurd. 30 years later, Charles Darrow played the game, loved it, and took the idea to the Parker Brothers firm. They mass produced the game under the new name of Monopoly, and Darrow is remembered as the founder of this game he actually never created.